I have no experience with the proposed lens though my jaw dropped to see 2600mm for $250 thinking "wow it would sure be nice if that was really a decent lens, complete with functional AF and nikon mount and a tripod mount that actually worked and enough shake-damping that it would be useful....."
As it is, I doubt that most of those criteria are satisfied. Often even if the lens is AF there are problems focusing at low light levels (which are almost always low at that FL even in noon-day sun) or high FL settings due to the design issues with the camera / lens. Shake and mounting rigidity are serious issues. Ruggedness and resistance to dust / dirt / humidity / misalignment / torque & flexure are others.
The telescope + t-mount + camera is in theory a very commendable solution with the exception of one huge problem -- lack of auto-focus and difficulty of using the viewfinder to focus acceptably well. If you can get it focused properly you can take some very lovely pictures through a good spotting scope (somewhat rare) or especially a good moderate sized telescope.
Avoid schmidt cassegranian mirror + lens designs and lenses with F/ratios less than F8. Having a high F8 or longer F ratio makes focusing MUCH easier and reduces your severity of coma / abberrations and focus distortions across the field of view (it could be somewhat sharp at the center, and very fuzzy near the edges).
The absolute best situation would be a quality refractor telescope with as big of an aperture (3", 4", 5", whatever you can afford) as possible and the typically long FL and high F/ratio those have when they're good.
The next best would be a newtonian reflector at F/8 or longer, maybe a 4.5" or 6" or 8" aperture instrument (it doesn't sound like you needed portability if it is right in your back door so to speak), and a tube length in the 3'-6' range depending on aperture (i.e. F/8 or longer).
Avoid the Maksutov-Cassegrain C90 and similar scopes with relatively short tube lengths, and a combination mirror + lens optical system. They can be OK for visual observations, and the waterproof / lightweight aspect is nice for hiking, but as a telephoto you'll find sharpness lacking and focus problematic.
The problem with many DSLR cameras is that they don't offer particularly big / bright viewfinders, don't offer sufficient viewfinder zoom-in attachments for precise focusing, and don't offer a good manual focusing target such as some of the nice split ring type focus systems on high quality old flim based SLRs. It is often impossible to get the focus sufficiently close to right with many such DSLRs in manual mode, especially if an awkward angle (telescope mount) and low light (high FL) situation with lots of vibration (small tripod, wind, you touching the thing at high magnification) situation prevails.
It is possible to use the AF indicator even in MF mode on many DSLRs, so sometimes if your focus point has some kind of high contrast well defined pattern to it you can just look for the green dot, though shake and such factors can still hinder this, and having a low contrast target field ruins the scheme.
If you have a laptop / computer that can control the camera to do exposures via remote capture over a USB link and get immediate PC based previews / reviews of the shots, that can help tremendously since you can dynamically (manually) adjust the focus while getting a fully usable feedback image within a couple of seconds to indicate the quality of shot you're getting.
If you just need moderately better telephoto, you can always buy a decent 1.4x or 2x teleconverter adapter for your favorite quality 200mm or 300mm lens if that lens is more than satisfactorily sharp at that FL to begin with since of course it'll only get worse as you pile on the zoom. AF / light level / vibration related limitations will increase of course.
I'd go with a good full on telescope and a good sturdy telescopic tripod and t-mount for the best results for the least cost, and plan to use a laptop to control / review / optimize the shots in real-time.
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