If you want one that folds/ is more compact, then
I highly recommend this treadmill. It's really good quality, has awesome reviews, folds up AND you can hook speakers to it! The price is right too - very budget-friendly.
As for the treadmill desk idea.... from what I've heard, you either love a desk treadmill or hate it. There doesn't seem to be an in-between. If you only plan to RUN on your treadmill, I'm not thinking you'd be doing much desk work at the same time - just something to think about.
However, if you DO decide to get a treadmill desk set-up, I'd totally
get this TrekDesk Treadmill Desk... it's really great and has a price that is more realistic for "regular folks"...lol!

Best of luck!