Well if you are really starting out, then the B4 will be your best bet as you can find parts for it all over the place.
And if you don't mind ordering parts for the RB5 then I would consider that buggy as well, how ever set-up info will be to a min as the car is still newer (although Casper, and a few others will help you with a great starting set-up)
The Losi is ok, and you should be able to find parts for it at some hobby shops (also I don't think you can use very many parts from the xx-4 on the XXX-cr)