I own a Singer 29-4 like this:
https://www.google.com/search?q=sing...w=1333&bih=645
It would make an awesome long-arm except the bobbin only holds about 36" of thread.
The 29-4 is an interesting machine as it does not have lower feed dogs. The foot acts at the feed dog. Replace the foot with a hopping foot and I'd have an awesome long-arm - but - I'm still limited in the bobbin capacity.
Oh, forgot to say, my 29-4 can be either treadled or run with a motor.
Shari