First welcome to the community.
Next, personally I would be afraid of the deal. If it has a bad head gasket you probably know there is the risk coolant is in the oil and thats bad for bearings. I cant say I have ever heard of a cracked block in the 3800 community, but that doesnt mean it hasnt happened. Additionally, if it is an SE, doesnt that have the 3100 engine? If so, and it has a bad head gasket or bad intake gasket I can attest to the damage it can do from the experience with my wifes Grand Am with that engine. We saved hers in time and it went on to provide a long time of service but it could have been soooo much worse.
If you were set on getting it, you could probably get a motor from Ed Morad (Morad Parts Company, a GP community used parts specialist) for very little. Heck, I dont know how possible it is, but you could probably drop a L67/4T65E-HD combo in there and have fun, I dont know you can do that for fact. Im working from the assumption its a Grand Prix SE with a 3100 engine.
Personally I think you could find a better Grand Prix trade else where, but thats just my opinion.
