[chumbucket]

The SeaDrives were a good concept when they first came out. Then the introduction of the outboard brackets for universal mounting came along and were the demise of the SeaDrive. I thought they were 1.6 liter on the smaller ones but could be wrong. I thought i was wrong once before but was mistaken. ;D They make about 115 Hp. The heart of this motor is essentially Johnson/Evinrude and isn't much of a problem to buy parts for and keep running. The questionable part of the package comes with the mounting bracket which incorporates it's own tilt/trim system. Anything to do with those components (ie; pump, cylinders, hoses, etc.) are no longer available. One might get real lucky and come across some parts, but not too likely.
IMO, this would not scare me away from buying the boat for the right price. Because, as MJ said, you get a full transom boat that is very easy to repower later on with a bracket. Minimal work involved since all the hoses and controls are already routed through the upper part of the transom.
Use the fact that it has a SeaDrive as a bargaining factor to push the price down. The owner must surely realize these things about that motor so I would think you could come to a happy agreement. Then, if all went well and you bought it, I would run the motor until it died. In the interim, save your pennies for a future repower. I also feel that 115 Hp is very minimal for a V20, so you're going to want to go bigger someday anyways.
Your Explorer would have no trouble at all towing that boat.
Best of luck whatever you decide. 8)