"dumb woman" needs advice

This past March, as I do every year, I had my Weil Mcclain boiler cleaned...it is approx 25 years old. It's been working fine. The tech told me there was a crack in the "chamber" and that I could get the whole chamber replaced..he said I could probably go another year without doing anything, but I should think about it. I called my oil company the next day, and they gave me an estimate for the repair of approx $500.00. I told them I would call in late summer to have it done. So I called, and all of a sudden the guy is hemming and hawing about the repair and he wants to come out and look at it and I think I'm going to hear that I should replace the boiler. Can't. Don't have a job. Unless it is a total emergency and the thing craps out.
My question is, what do I ask him so he doesn't try to pull one over on me like trying to sell me the whole deal and not just fix the problem?
BTW, it is NOT leaking as far as I can tell.

this is why they say get 3 bids. so get 3 difference company's to take a look at it and see what they say.