After some searching I have narrowed the alternatives down to:

<url snip>

Any views on this, what should I go for? Or maybe you have a better choice?
Will these alternatives be able to handle a forum with up to 400 online users and 900 daily posts?

That is too much for VPS.

You would need to discuss this on another forum outside of IPS as they offer their own hosting solutions.

I've removed the URL's to two other hosting companies as it's competing services :)