Cheapest accomodation would probly be one of the backpackers in Paia, get a safeway member card for cheap food, fly hawaiian cheap and easy on excess baggage.
Sleeping in a van is OK if you can find the right spot (away from the crack heads

) but this is not recomended as you might get in alot of trouble

Theres heaps of cheap cars to buy over there not sure about insurance though. You can get rental cars for as little as 25us a day cant remember the name of the place but its in kahalui someone will know cos heaps of windsurfers use them.
July and august are best for wind but there wont be any waves. Spring or autumn give a good mix.
Getting a job should be alright I worked as a labourer for the stones when I went last year cleaning up rubbish on a building site hard work but good tax free money

.Lots of building going on and they were always keen for hardworkers.
You will have heaps of fun so book your flight know!!!!