I think $300 is too much for a 15yo bike that listed for $440 new. It does have some nice features like that 8sp nexus hub, front springer fork etc. and it's a Trek. Dunno, how bad do you want it? I'd offer $150 and see where it goes. Remember that bike is not a good candidate for an inframe motor because of the space in the V.
Just my opinion, but I dont think a newbie should be putting a cheap engine into an expensive bike. The engines have a lot of power and will destroy your rear wheel if not aligned correctly or if the rear wheel is not durable enough.