This thread answers most of your questions.
My advice is to make sure you go for a facelift - easily distinguished as the pre-facelift has two white sections in the rear lights and the facelift has a single clear (or darkened) section and three instead of four blades in the front grille (of course someone may have upgraded these)
There are a number of other styling improvements especially to the dash and it is generally thought that the facelift is a much more 'sorted' car. If possible go for the AMG sport. This styling really adds to the look and also has some nice extras.
Essentials:
Leather
Metallic
Auto
Really good to have:
Comand (a bit dated now though, but always a bargaining point)
Parktronic
Heated seats
Xenons
The roof is a complicated piece of kit - around 1800 parts iirc but seems to be generally reliable - I did have one issue with mine where the rear deck didn't shut when the roof closed, but this was sorted by lubricating some of the many moving parts. I think that the root cause was that the grease had hardened (and gone sticky) in a couple of locating pins and they weren't being guided in properly (a bit hard to explain)
Prices seem to be holding up well so I think you might be a little optimistic on your budget to find a really good one, but they are around at that price. Also, this is a good looking Mercedes convertible - when you come to sell just how much can you lose?
Anyway, while you are deciding, here are a couple of piccies to whet your appetite.
hth
Edit: Oh, and if you can, get the genuine wind deflector. Easy to fit and stow away and makes a huge difference at all speeds. Even gets the Mrs Ted seal of approval and that is saying something.