It took me 4 months to find the CPO 135i convert 6mt I bought. I picked mine up at the end of last summer (first week in Sept).
A couple things I did in my car search (in addition to visually inspecting the car)
1. I ran a carfax on all cars that I was interested in (review for accident history, number of owners, BMW buyback from unhappy owner, etc.)
2. When shopping at BMW dealers, I let them know that I wanted a printout of the service history for every used car I looked at. (The car I bought was a 3yr 30K mile lease return, HPFP replaced 6 months from new, all service/work performed at original dealership, one owner, driven most in summer months)
As far as buying an 135is or 135i, I would say buy an 135is if you like the rims and are not likely to buy an aftermarket tune. Otherwise save the money and do the mods that you want (thats the route I took as I new I was going FBO plus upgraded suspension and wheels anyway) The only 135is I drove was a coupe, I liked the exhaust note alot and did not find it too loud or to drone too much.
Good luck in your search. IMO the 135 convert with a 6mt is an excellent combination and I have no disappointments
