I sailed a MacGregor 26-D before I moved on to a keel boat. Using a Mac as a power boat isn't a bad idea but there are some points to consider.
You can use it without putting the dagger board or swing keel down even if operating with sails. It won't sail where you point it unless that's downwind. But board up is the preferred mode if motoring.
There has been at least one capsize with fatalities of a Mac with the mast up, the ballast tank empty, and if I recall correctly about eight people on deck. In short Macs can get top heavy if the ballast tank is not filled completely and closed.
The older Macs can only take about 10 hp motors and will cruise only about 6 mph. More horsepower will not make them go any faster because the bottom is not flat enough to allow planning. It would just push up a bigger bow wave and probably become unstable.
The later model Macs, i.e. the ones rated for 50 hp motors, have a flatter bottom so that the boat can plane. Don't expect them to go much more than 20 mph even with an empty ballast tank and no mast.
Boats depreciate the most during the first ten years. After that prices stay pretty flat with condition being the most important factor.
Don't dispose of any sailing hardware or the resale value will plummet. Sailboat hardware is expensive and problematic to replace. Check the price of a winch here:
http://search.defender.com/?expression=sailboat winch
If you buy a boat missing
any topside hardware the boat should be
very cheap. I would not pay more than $2,500 for a stripped Mac even a late model. You mileage may vary.