Hard to realy comment on that, Variable speed is nice, but for most users, it's turned on full speed then locked in, so it ends up being high speed all the time, look for one with a dial type switch control, normally located near the trigger and numbered, that way you can set a speed and lock at that speed.
Weight is hard to comment on also, I like a heavier tool because it gives you better control, but if you are holding it overhead it gets tiring, so you'll have to compare them and decide which feels right to you.
Take a trip to a few shops and see what sizes of bonnets are available, and then try to get something that size, some buffer/polishers have odd sized bonnets and you either go through that company to get replacements or make your own.
For most tools I take a look around town, then check out reviews on line
, pick out a couple that have decent reviews and then compare them a bit better at the local stores, unless the item I'm loking for is impossible to get otherwise, I always buy local simply because I like having what I buy in my hand when I pay
I used to say porter cable was the best, but black and decker screwed them up, so I can't say now, but a few of my newer tools are hitachi.