(Update 3/13/10) It's been 1 1/2 years since I've announced this project, and I've decided to revive it. Now I just need some cash.
Bionicle,Neo Shifters,even the energon connectors. The arms should be the easiest sence they would be a solid arm with the cars simply attaching via magnets.
Sounds great! Have you considered using the Hot Rod mold so you'd have 4 different cars? Are you basing the heads off the pictures you have posted, which all look almost exactly the same or the Cartoon ones? Either way good luck with the bash. I look forward to seeing your progress
wait, wait... if you're using Classics Prime for your Motormaster, why do you need Unicron thighs?? I also think you ought to use the limb-bots' legs 'n waists for your arms (like I did) as that's not only how Menasor IS built, but then you can avoid magnets and it'll be more accurate.