The typical ScubaBoard BP/W solution works well here. I will offer slightly different advice.
I have exactly what some people have advised--an aluminum backplate for travel and a steel for colder water, but you obviously want to start with only one. That makes sense. In that case, get an aluminum back plate. It is lighter for travel, and it packs up in much less space than a typical BCD. You are going to have to add weight whether you are diving in a tropical location or a local, colder location, so the only difference will be that in the colder location, you will have to add a few pounds more than in the tropics.
If you end up using heavy steel tanks in colder environments, you might not need much more eight at all.