I built the Dayton BR-1s a few years ago as my first kit. They sounded pretty good, but as I've gotten more into 100% DIY stuff, I've found that I like a lot of other designs a lot better. The TriTrix that Plungers mentioned is a design from the DIY community and is REALLY popular. For the budget, I'd recommend that one as well:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-702
The only downside is that you'll have to solder the components. Soldering is dirt-simple though - the Dayton's were my first real soldering project and came out fine with a $10 Radio Shack iron (and lots of tutorials are available online). Plus, Parts Express has a Youtube video on how to assemble the crossover:
http://www.youtube.com/watch?v=H4Y1OGalDUo
If you don't want towers, also like Plungers mentioned, the Overnight Sensations are another popular bookshelf-style design from the DIY community:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-706
Here are some other source for DIY kits:
http://www.diysoundgroup.com/
http://www.madisound.com/kits/
For gluing the cabinets together, I'd recommend running down to Home Depot or Lowes and getting some PL Premium construction adhesive from Loctite. You'll need one of those $5 caulk guns. It's pretty strong stuff:
http://www.youtube.com/watch?v=KDe1KwWH6qA
There are lots of options for finishing the cabinets, not sure if you have a plan for that. Probably the easiest thing is truck bedliner in a spray can - nice textured black paint that hides any defects. Rustoleum & Krylon also have a lot of cool spray paints, like hammered, camo, and stone, as well as the regular metallics and matte/glossy colors. I'm doing a 7.1 set for my brother right now with Rusoleum's Hammered Green on the faceplates, which looks pretty cool:
http://www.storesonline.com/members/1506961/uploaded/hammered_green_closeup.jpg
MDF is very porous material, so I would recommend sealing it before painting. I use a couple coats of Zinsser B-I-N shellac sealer/primer (also available at Home Depot etc.) with a foam paintbrush, then sand it down. I also use wood putty (Elmer's sells small tubs of it at Home Depot) for sealing any cut/curved edges before I put the primer on, because they will suck in paint like a black hole unless you seal them. Here's a link to that BIN stuff I use:
http://www.rustoleum.com/cbgproduct.asp?pid=216
One thing I've been using lately is contact paper (like for lining the insides of cabinets) - you can get faux wood, granite, marble, leather, etc. I'm doing a pair of stereo speakers for my cousin for her wedding in faux brown leather and white paint and they are looking pretty awesome so far! Another Home Depot steal, haha. If you want some higher-end stuff, there are some nice vinyls here in different metals (stainless steel, etc.) and different granite colors: (they sell them on eBay as well)
http://www.stainlesssteelfilm.com/main.sc
DC-Fix is another cool brand (a lot is available on eBay) that comes in faux woodgrains and stuff:
http://www.berlinwallpaper.com/dcfix/
So you can get some really nice-looking woodgrain on your speakers without having to use veneer:
http://www.berlinwallpaper.com/dcfix/images/apple-birch.jpg
All kinds of stuff to play with!