In reality, 30mph is pretty damn fast - you can't maintain that for long on flat ground so you are really only talking about limiting speed on downhills. You can have a lot of good long rides with a max pedalling speed of 30mph. And if you can bring your cadence up you will be faster when you eventually get a road bike.

So for now, consider training on your Schwinn and saving $$, spend only on necessary items, keep an eye on craigslist for a half decent road bike that will fit and that matches the money you have - be sure to budget for dedicated shoes and clipless pedals as they make a difference in performance on longer rides.