Early March would be OK for Bishop, but you don't really want to be there any later than that as it'll be too hot for a lot of stuff. It is an amazing destination and the Buttermilk are my favourite bouldering destination that I've visited worldwide.
Accommodation is pretty much as cheap as you want it, the Pit campsite is $2 a night for a pitch, has long drop toilets but no running water (but you can get water free from all the gas stations in town). There are a variety of slightly more upmarket campsites (showers, etc) that will be open by that time of year, and if you have a bit more dough to spend, it's usually possible to get deals in the motels in town (around $50 a night usually but you can normally get $200/week or thereabouts).
People climbing at your sort of grade usually seem to prefer the Happy and Sad boulders to the Buttermilk, and the sport climbing at Owen's River Gorge starts from quite low grades. I recommend checking out the latest bouldering guide when you are in a shop or at the wall, one of the most inspiring guidebooks I own!
EDIT: and what Chris said.