Buying now with a future upgrade is no problem! I personally never recommend an upgrade install but it is VERY easy to perform a clean install using upgrade media. Just use the search function here on the site to search "clean install using upgrade media" without quotes, it may even be stickied in the installation forum.
I would personally recommend buying the laptop you can upgrade to Seven rather than one that is pre loaded with Seven PLUS megabloat from HP or Dell or whomever. If you buy a Vista laptop now and order the free upgrade later when it is available you will have a clean system minus the bloat put on there by OEM's who get paid to put it there! It's a win win situation in my opinion of course!