Have you looked at Z-brush? The price is in your budget and it should run okay on your system. The interface is a bit different from most software (it's French) but once you get past that, it's a very powerful program, which a lot of people are using effectively. There's no free trial, but take a look at some of the tutorial videos in the "Z-classroom homeroom": http://pixologic.com/zclassroom/homeroom/
to get a feel for it. I'd say give yourself a chance to get into it, though, before trying to use it for a real presentation.