150 active users is a pretty good size forum, I am surprised it took this long to upgrade to a VPS but definitely a good choice. I would recommend going with Centos if you plan to go with cPanel as that is what is supported by them and most any provider.
2-3gb is a good place to start but I would say more around 4gb, I would push to the higher side if possible, especially when you peak to 150 users online at a time you will be glad you have it.
Space is dependent on you, whatever you actual needs are. Just take what you are using now and notch your requirements up about 40-50% and you should be in a safe boat.
DA is good in comparison to free panels but considering DA can cost just as much as cPanel I would definitely suggest going with cPanel. Its just overall a rock solid platform to use, its supported by just about anyone and there is a million and one how-to guides to learn from.
Litespeed is an additional $14 from LS them selves, $19 I think for the one that supports more than 2GB ram VPS which you would need so make sure you need it. All in all though your budget should give you plenty of options and I would suggest browsing around the offers forums to just get a feel for things for now and compile a list of providers that stick out to you.
I've been running a forum with about ~300 active users using IPB software for about a year and a half now. I recently made the switch to WiredTree and couldn't be happier.
I am running their SSD2000 plan which is more than enough to suit my needs at this point. I use about 100GB of bandwidth monthly and am nowhere near my limit. The cPanel license is also included with every server, so that's a bonus for me.
Obviously your mileage will vary based on how many active users you have and your particular software, but those are my two cents