Deciding on tires at this point, still have an open diff. With an r1 motor what width tires would you recommend. I have read that becs are difficult to launch, then why not go for slightly wider tires, or is it a matter of weight over the rear wheels (although my BEC is still a little rear heavy with driver)

No, when people say "hard to launch" they mean that, due to lack of a flywheel, it's very tricky to launch smoothly without stalling the engine. It's a learned technique, but if anything, wider tires will make it worse.

185's are fine on the rear of an R1 BEC, as are 205s. I wouldnt go any bigger than 205's personally