You can look now to rent for next summer, but Boulder isn't exactly a one horse town. In other words, there are plenty of places available no matter when you look. Search Craigslist, etc.
As for your questions:
The effects won't be all that great unless you reside even higher than Boulder, which sits at 5,400-feet. The most bang for your buck starts around 6,500 feet, though most athletes aim for closer to 8,000 feet, a good compromise between benefit and risk (primarily in the form of too slow recovery). As far as "coming down," the effects last up to 3-4 weeks but it's generally best to come down 14-17 days before your biggest event. Ideally, you don't return to sea level straight before an event.
Re: Is there any benefit of just going for a few weeks or would that just be a waste of money?
A few weeks is the minimum for the desired effect. After that though the gains tend to be less pronounced.
I think it's a smart move if you can sleep and train at 8,000 feet, but if not, you'll still get some benefit. 95% of all medalists at the world championships and the Olympic Games since '68 have either lived or trained at altitude.