Generally either soaked lucerne (Alfalfa) or grass hay with certain types of grass (Off the top of my head I think you want to avoid Rye Grass? But that's here in Aus so you may be different).
Lucerne is actually relatively low in sugars but is high in protein and can throw off your Calcium/Phosphorous ratio.
Agree with others though - Focus on buying a good, sound horse, then deal with what type of management it needs once you get it!
I would never, ever buy a horse with a known history of founder. Too much trouble and heartache. In my opinion it is easier to feed more to keep weight on than to try and keep weight off.
Again with a known history of colic - Once or twice, very mild colic would be ok, but any more than that I would pass.
In 10+ With probably 10 or so horses we have only ever had maybe three cases of very mild colic, all which passed without the need to call a vet. Colic is usually a management problem and shouldn't happen very often (Of course there are exceptions - But I wouldn't want to buy one!).