I'm in the market for a Z3 M, I have questions

I'm now looking at buying a used Z3 M Convertable 5 speed (can't afford the 6 speeds). What are some of the know trouble spots on these Models? What are some known problematic year models to watch out for? Whats the average feqnecy of repair, and what are the average repair costs? What is the typical price tag a 2000 Z3 M with 53K miles w/ nice custom exhaust & factory premium wheels? HAda stealership quote me an asking price of $24.5, whihc seemed low according to online places like KBB.com and intellichoice.com. Please enlighten me with some sound advise that I shold know as a prospectable buyer. Thanks, Jay