Well, since you are looking for a 1/4 scale then I'd say yes, you posted in the right place, but I promise you that here in the Sanctuary we really aren't that fussy about where folks post. I rarely move things and only if I think they'll get better viewing elsewhere
In answer to your question
1. Just depends. You can buy doll heads from the manufacturers, second hand on various doll forums, ebay, etc... The real question is more whether a resin color will match the bbb body you've chosen or not and if it will fit. And again, that just depends.
2. Any doll can really wear another company's head, it's more the resin color match that can be an issue. BBB white, for example is REALLY WHITE, whereas Angel of Dream, or Dollzone are more eggshell white. Some companies have Pink Normal or Yellow Normal etc... Probably what you'll want to do is find a head you like and then see if it will work on a BBB/RS body. Or just buy one of their dolls complete - they have lovely faces!
3. BBB/RS are notorious for being floppy - however that is something easily solved by restringing (they don't use very good elastic) and sometimes also "sueding" - putting a bit of hot glue into the joints to increase friction and pose holding. that said, I really don't like how the MSD boy body (in particular), when you sit it down has the knees knock together and the feet spread apart
I rarely let my BBB boy sit because of this.
4. Since they pretty much all have the same body, it's all a matter of face. Mei is one of the most popular, and if you don't like pointy ears then she comes in a human ear version (whose name I forget).
5. MSD - but that said every maker is different so unless you buy something that has been specifically made for a BBB/RS you may find you need to shorten, or take in or take out etc... Every maker has different measurements so there is always a little give and take on clothing.
BBB/RS are wildly popular for their beauty and their exceptionally fair prices. Great intro dolls for those new to resin and almost everybody except the most snobbish collectors seems to own one
I think I have close to 10.