I have a trek 850 that I put tread on the back that was less nobby, almost bike path type of tread and I put wheel on front from a hybrid bike. It has no fron suspension. Seems to work fine on lime stone and really goes well on paved bike paths. Theres are easy, cheap mods you can do yourself. Just go to your local used bike store and buy a pair of forks and rim/tire (or take them off another bike). You wont need much more than a wrench and an allen wrench.

Used forks should be less than $20, should be able to get a used rim for $20-$50 tops depending on the quality/condition. That'd be a lot cheaper than buying a new bike. Might be extra cost if you need different headset but they are not expensive and tires and tubes are fairly cheap.