Don't know if your building your own platform or buying one, but make sure it's very stable. I built mine (but really did have to put it in a crappy place) and it wobbles just slightly, but it washes a lot of speed off the bag...
Knowing how high to pump it up is pretty easy, pretty much until it get's hard. You can start of pumping it up so you can still press fingers into it slightly, which makes the ball bounce a little bit slower and good if your not that experienced on it. Then when you want to speed it up, just blow it up a little bit more.

Also, you don't have to go big and expensive on the swivel... I had a moulded plastic one that came with my speed bag, but thought a better one would be uhhh better, so I bought the $30 steel one off ringside.com.. After a week I switched back to the plastic one that came with my bag. It was much faster and the bag felt much better with it... I think it's because the swivel is quite compact and leaves the top of the bag close to the platform, making it bounce a lot faster with less path for the swinging.. I've used bags with other swivels but honestly think this is the best one around...
http://www.jimbradley.com.au/html/s1...v_top_id=Shop2 that's the link for it if your intersted..