If you've ever done Technical Drawing or whateever in school, and done a bit of CAD work, I'd recomend Rhino 3D. Its not much of a rendering program -use truespace 3.2 (free=good) for that- but Rhino is much easier to use than truespace for modeling, because its interface is a lot like a cad program (ie autosketch). In general, to make an object you create a curve, then interact it with other curves - there's lots of options and techniques to do this, such as revolve, rail sweep, lofting. Once you have a few surfaces, you can get solid objects.
After about 1 hour in Rhino's tutorials, you should be able to put out a reasonable model.
Though there's a lot to learn, i've been using rhino for a year and still don't know how to take advantage of all the features.
The evaluation for rhino is free:
http://www.rhino3d.com/download.htmGenerally, I build my models with Rhino then render in Truespace. Truespace has more detailed lighting and texturing options.
If you just want to render freespace stuff then you'll just need Truespace. After you've rendered the models, you'll want a program like Paint Shop Pro, to add things like background nebula's, weapons fire etc. PSP takes a while to get to know well, the layers feature is a bit confusing at first, though extremely powerful once you get to know it.
I'm not sure how much PSP costs - my dad got it from his work - but its probably quite a lot.
