Why not build on the Exacta since you already have a start? Or, since you have the Contax, pick up the FX-3 or FX-3 Super as again you already have an investment in the lenses. Or, since you have some M42 lenses you really like, why not get a body that can mount them directly such as an Asahi/Pentax? One problem with buying SLRs of varying make is you end up duplicating too many accessories and lenses rather than having the depth in on system to make it truly universal.

Maybe I am a simple person but years ago I decided I hated duplication where ever possible and settled on as a main system a Bronica ETRs that I could use both as MF and 35mm. I dumped everything else for the most part in trade. Having 5 lenses I can use on either my MF or 35mm camera system really saved a bundle. Yes, since I've added a Leica but I could afford it as I'd stayed clear of having multiple systems. I've found the key to systems is rationalization. Now, to put away my soap box.