For the majority of the car (panels / trim etc) the build quality (solidity) remained the same all the way through. The later cars had revised interiors with better electrics and more features available - Brake Assist, ESP, Panoramic Roof, 5-speed auto - the very late 500s had a lot of options as standard - Nappa Leather - Xenons, 18" wheels, electric folding mirrors etc.
IMO the things to be aware of are engine and transmission changes, wiring harness improvements, soft top revisions and introduction of Mobilo Life. There are alot of interior changes - aircon updates - CAN bus introduction - OBD2 etc etc - if these are important to you then consider a later car.
The M119 V8 went through a lot of revisions itself - becoming distributor-less and moving to coil packs in '95 - about this time it's safe to say the wiring harness issues were fixed (with MY1996). Water based paints came in from '96 onwards with revised galvanising of sheet panels - though Mobilo wasn't introduced until Sep '98 with the introduction of the 1999MY.
You can't really go wrong with a 1995, 1997 or '99-'01 car - that's not to say the other are bad - take each car on it's own merit - just that these years specifically ironed out problems of previous models.
Buy with your head however - there are some hideous examples out there - interior trim is very expensive to replace, the soft top operation and aircon on earlier cars is costly if more than just a dodgy switch - a "£50 re-gas" is a common way of saying the aircon is buggered - if it's so cheap - why haven't they had it done?
Prices obviously vary accordingly - a mint early car with FSH and low owners, lowish miles expect to pay £12k - say £15k for a '97, £17k for a '99 and anything from £19k - £25k for an '01.
Good luck with your search!
