I've been shooting the 645N and NII for the past year and a half. If I had to sell all my cameras but one that would be the one I kept. The plain old 645 has a few disadvantages, to me the biggest one being it uses buttons instead of dials.
Check out Pentax Forum for reviews and Ken Rockwell -- whatever your opinion of him might be --- has good reviews of the 645 and 645N.
In my opinion the lenses are excellent and my cameras have proven durable. I do a lot of mountain biking, hiking, skiing, snowmobiling with mine and it's been rugged.
$299 is a lot to pay for a 645 body. For that kind of money you should get at least a body + lens or else a 645N body.