I'm not sure if you would want to rely on it as your ONLY transportation, but a lot of people on this site do that. It is, afterall, a 13 year old convertible. Even with my 6 year old low mileage z3, it leaks a little bit.
The Rack and Pinion... never actually did it, but if it's anything like changing tie rods and the like, anybody who is handy with wrenches or the ability to read the internet and jump in and learn you could do yourself.
I wouldn't go anymore over 4.5k as the starting point. Look for 4k if you can, but if it's nice 4.5k...