Good choice. Nikon > Canon right now. Better sensors (kickass DR), and better build quality on their products, especially their consumer grade lenses which Nikon warranties for 5 instead of 1 year, and gives you free lens hoods on everything whereas Canon is too cheap to provide them for free on consumer grade lenses.
If you are just starting out I would suggest the D5100 unless you know you will use the advanced functions of the D7k. The D7k has a long list of bonuses, like better AF, metering, larger viewfinder, etc. but those are improvements from something to something--and the d5100 is a whole lot of something.
The improvements from nothing to something are no commander mode to commander mode, no internal screw motor to screw motor. No DoF preview to DoF preview. No guidelines in viewfinder to guidelines in viewfinder (optional).
If you have enough time before your trip, then you might want to use the kit lens for a while until you get a better idea of what you really need. After a while you will know if you are:
- happy. Then it's end of story. No other equipment needed.
- want more range but don't like changing lenses. The best option is either the 16-85 VR if you care more about the wide end, or the 18-105 VR if you care more about the long end. They are both sharp and good. The 18-105 VR's plastic mount is less of an issue if you never change lenses anyway. Also consider a superzoom like the 18-200 or third-party superzooms, though be aware that superzooms are only sharp on the wide end and lose sharpness as you stray longer, faster than dedicated telephoto lenses would.
Note that superzooms and telephotos do okay with portraits, generally. Just step back, zoom in as much as possible, and open up aperture as wide as possible.
- need more low-light capability against non-moving targets. Get a tripod if applicable. Else, a 17-50mm f/2.8 Sigma OS lens (it has image stabilization) will give you better results and zoom range at the same time, than any of the large-aperture lenses. Remember that your depth of field narrows and you lose sharpness as you open up a lens, so even a 35mm f/1.4 lens is less than idea for low light, whereas the Sigma will be fairly sharp at f/2.8 with a decent depth of field, yet with its good image stabilization will give you THREE EV stops more effective exposure time. Furthermore, 50mm at f/2.8 on DX is good enough for portraits and imho an easier focal length for portraits than 35mm on DX. If you try the same stunt with a 35 f/1.8 you will get more perspective distortion (this is the "big nose small ears" effect you get when you photograph something up close). You need to back off some... in fact it'd probably be better to back off with a 35mm at f/1.8 and crop it later, even if that amplifies the perceived depth of field.
- need more low-light capability against moving targets, including people indoors. Use the onboard flash, and if that's not enough, get a cheap but good one like a Yongnuo 465 or 467. Use a flash before you resort to the 35mm f/1.8 which everybody gets (myself included, though frankly I would not have bought it if I already had my Sigma 17-50 OS, and I'm now looking to sell the 35/1.8), but few buyers actually use because it's just one focal length and not as convenient as zooms and not long enough to get depth of field effects as well as longer focal length lenses. (I think Thom Hogan made a joke about that once... "everyone buys it; see anyone using it?") If you can't use a flash, then yes, get a 35mm f/1.8. It's decently sharp wide open, arguably better than most other lenses in its class, including some pricier lenses.
- need telephoto in daylight. A basic 55-200 VR is relatively small, cheap and effective, though not particularly fast. A 70-300 VR is more expensive but has more reach... but weighs more. (A good alternative is the Tamron 70-300 VC which is almost as sharp and built much better than most Tamrons and maybe even the Nikon 70-300 VR itself. The Tamron has better VC and costs less, too.) A 55-300 VR may be a compromise you can live with. All of these telephotos will become gradually less sharp as you reach their long ends, even with stopping down to f/8. You can "overbuy" by getting, say, a 70-300 and shooting only up to 200mm, rather than buying a 55-200mm VR and shooting at 200mm, and thus get a bit more sharpness and larger aperture (at 200mm) that way, but whatever you do, compare sizes and weights. Preferably in person. Long telephotos are big mothas, even the smaller-apertured ones like these that only do well in bright light (because in low light you would have to crank up ISOs too much in order to get acceptably fast exposure times... you will usually need to hit ~1/500th second or faster against moving targets as VR will not help you much if the target moves.)
- need telephoto in dim light. These are out of your budget and hulking monsters. Nikon 70-200mm f/2.8 VR, or VR II. But if you just can't bear the thought of increasing your ISO any more to capture telephoto shots, get one.
- need a wideangle lens and can use a tripod in low light if necessary. Sigma 10-20 f/4-5.6 (not the 3.5 version which is no sharper but more expensive) does well stopped down to f/8 or lower and is the best budget choice. If you don't mind having to crop out nasty corners a bit, the Tamron 10-24 is the other good budget choice. I can't recommend the Tokinas due to ghosting/flaring issues though if you don't mind that, get one anyway. After that, prices go up quite a bit as you enter the realm of the Nikon 10-24 (newer, bigger zoom range) and 12-24 (still good, and better built).
- need wideangle in dim light and tripod would not help. Tokina 11-16/2.8, despite its ghosting/flaring issues. No other real choices as the only other plausible choice under f/4 at your budget is the Sigma 10-20/3.5. You will have to manual focus if you get a D5100, but it's a wideangle with distance scale and huge DoF, so it's easy.
If you don't have enough time to try gear before your trip, I would go with the Sigma 17-50/2.8 OS (great all-purpose lens even in dim light, popular with wedding photographers who need speed + low-light performance and can't be switching lenses all the time) and add a small telephoto like the 55-200 VR for things like zoos. I'm assuming you don't want to carry TOO much weight while traveling.
Good luck!
Edit: I found the Thom Hogan six word reviews on Nikon gear. They are here, and hilarious:
http://www.bythom.com/sixword.htm
after a lot of googling I am leaning toward a Nikon D7000 18-105mm kits ($1500) + Nikon 35mm f/1.8G ($200) , and around $100 for a camera bag, screen protector, UV filters, CF cards, wireless remote, etc
or the D7000 body only + Nikon 35mm f/1.8G = $1400, so I have around $500-$600 for another lense or 2, don't know which one to get thou. I will mainly use this camera for my upcoming USA trip (flying over from Asia for my sister graduation) to take picture of the family, street life, graduation ceremony and travel
also I will stay in NY so which e-tailers (Ritz, Adorama, Amazon n B&H) that I can order from without getting hit by sales tax ?
since I am getting the 35mm f/1.8G, do I still need to get the nifty fifty too ? 50mm f/1.4D used for about $250?