1989 GMC S15 Head Gasket

My truck is very high mileage (Around 150,000) and has a head gasket problem. I don’t think the head gasket is gone completely yet but I am leaking coolant from the side of the engine block. I’ve been thinking about replacing it myself and was wondering how complicated that would be. I do have some experience with changing out parts on various vehicles including the vehicle in question, but haven’t undertaken anything of this level yet, although I do have a good working idea of the process. I’ve had several mechanics offer to buy my truck because the engine is supposed to be un-killable ('Once the initial head gasket is replaced'). It should be noted the truck has a fairly recently rebuild transmission in it, and there are no major problems with it other then the coolant leaking from the side of the block (The coolant is coming from the seam where the top of the engine meats the block). Should I even attempt the replacement myself or should I sell it as is?