TS, actually camera preference can get very personal. Not everyone's needs and tastes are the same. I would encourage you to do your research online and, most importantly, go down to the stores to try them out. See how each camera feels in your hands, and if the user interface makes sense to you.
Things I suggest or things you should note:
1. As a new user, I recommend against buying used. You simply do not have enough experience to know if the used camera will be a good buy, or if it has problems.
2. Due to your condition, I recommend you get a good quality tripod that offers a detachable leg to be used as a monopod. You don't have to spend too much, maybe around 300 for an aluminium one. Look at brands like Sirui, Benro or if you are very tight, fancier.
3. There are a lot more camera models than Canon 550D and Nikon D90. Though that 2 models are excellent cameras, there are other excellent cameras out there. Looking at your budget, you should also consider the Pentax K-x (best value IMHO), Sony A500/550 and Nikon D5000.
4. Contrary to what you hear, not all current Canon lenses will fit all current Canon camera models. You can do a google on "Canon EFS lens compatibility" and you will get what I mean.
5. Contrary to what you hear, All Nikon lens in F mount from the late 70s till today will mount correctly on all current Nikon camera models. However, not all camera bodies will offer support for all the functions. For Ai, Ais lenses, they are all manual focus, and will only meter with D200, D300 and above. AF-D lenses will meter will all current bodies but will only support autofocus on D70, D80, D90 and above.
For your current use, I do not foresee much issues with lens compatibility if you go D90 or Canon 550D. If you go Nikon D5000, you will not be able to auto focus with some of the popular lenses like 50mm/1.8.
So there, I think that is a good enough overview for now. Feel free to ask more questions if any pop up.
