I'm in North Carolina, similar climate and building techniques to Alabama (wood frame on brick/block crawlspace). I've been renovating homes for 10 years and have run into termite damage quite often. Usually it is confined to one area--one room or one side of the house, usually one that is close to the ground. When the sill is damaged to the point you described, it has to be repaired. That's an expensive process that requires tearing off the siding, bracing the walls while rebuilding the sill, scabbing or sistering the old studs onto the new sill. Then patching the torn out wall and floor area that had to be removed to get to the sill. Then new trim, new siding etc. in those areas.
Termite damage like what you're describing is really, really bad, because you're looking at having to do the repairs on ALL the walls. Lots of time, lots of money. Also, if the sill is that bad, I would imagine the studs and siding are also compromised. I would guess that a contractor is going to bid you $20K at least. You might be able to get away with renting it without the repairs for a little while but eventually the walls and windows will start sagging where they are falling down because the sill is gone. And forget reselling it without the repairs.
If you make an offer on this one, I would be sure to have a contingency that allows you to get a couple of contractor's quotes on the repairs before the final decision is made. Be sure to factor in the vacancy time while the work is being done.