Lizzy -- If you go to treadmilldoctor.com or put in the address bar either "treadmill reviews" or "treadmill ratings", you'll get a ton of advice and reviews and prices of different machines. For the past five years I have been doing almost daily incline and speed intervals on my treadmill -- but it was a cheap one --a Pro-Form -- which has gotten bad press from others in these forums. However, mine has been wonderful -- just make sure, in addition to all else, the incline is automatic and it has pushbuttons for speed, etc., rather than a sliding bar. Sears or sporting goods stores should have ones you can try out to make sure you like it before you buy.
Also, if you use the search feature here or on videofitness.com you should be able to get a lot of comments on different types of treadmills.
Incidentally, this is one of the best investments I have ever made! Hot or cold outside, rain or sun, early morning or late at night, workout outfit or housedress and sneakers -- it's always available! Good luck!