Duuuuuude,
The Mexican residency wont mean much since it's been 16 years and it's been in annual since then and has logbooks. As far as the engine, I've seen a lot worse.
I'm not saying it's a steal. If you want a steal buy a 15,000 hour airframe, it'll still be safe if it's been maintained well. Notice that I said "with the right buyer."
Low time 152's are rare these days, long range tanks nearly nonexistent (although I doubt that will mean much) and if that's what you're looking for the price the owner is asking is within 10k of what you'll have to pay. I can see the "firm" stipulation making it not sell, and that's about it. I'm inclined to agree with the "not a motivated seller," theory.