I talked to a guy that has a B6100 HST that "needs a motor" He said his friend put gas in it instead of diesel and now it needs a "rebuild"
I'm kind of hesitant buying a tractor that doesn't run, what would a project like his be worth assuming that I will be doing the work myself?
According to tractordata the B6100 was in production from 1977-1997, is 14 HP, and has a .7L 3 cyl. diesel and about 1200 lbs. It's what I would call a Sub Compact. You didn't say whether it was 2 or 4WD.
Unless the guy is going to give it to you free, and even for free I think it would be a money pit to rebuild. There are tons of used running small compact tractors available. There are 4 B6100's on Tractorhouse.com ranging from $1500-3200 in running condition. Depending on the damage done I don't think you could rebuild for that price. Just my opinion.