I'd go with a 4th gen, but make sure you have it inspected with a fine toothed comb. You mean buying guide as in what to look for and what to expect? If so, the 4th gens have a chronic issue with seized pistons in the calipers. I have a 2005 and started to change my brake pads yesterday. My front left caliper had one seized piston and the right front had two seized pistons.
Aside from that, I haven't had a single issue with my 4th gen. I love this thing! So comfortable and well laid out. The 3rd gen feels too narrow IMO, but then again I'm a stereotypical American, used to stereotypical American oversized standards of comfort lol.
Good luck in the hunt!