I have a HE 5000 that is about 4 years old. I have run the pump 24/7/365 to keep water circulating even in the summer. I hav eyet to discover a leak, but from posts on here, I believe it is just a matter of when not IF. I have to replace fire brick every year, and they are not standard size, they are bigger. I would think the only way to check it out is to fire it and wait to see. Old ashes, creosote, stagnant water in the tank, grass growing under it, critters, missing chimney cap, etc. are all going to speed the corrosion factor. Mine can "supposedly" be repaired being mild steel or "boilerplate" . I am not familiar with the weak spots are yet for rust, and I hope I never get educated.
On the plus side, I really do like the design and efficiency of the 5000. Just wish I had seen into the future with the company. If you are looking to take this out of the equation for bargaining power, I would lean on all the posts from leakywooddoctor, once burned, green, hawken and logboiler. Any combination would have scared me off before I bought mine.
On the other hand, if anyone could point you in a direction of anew one or how to go about repairing the old one, There are numerous great minds on here to give great advice. Martyinmi,Scott7m, yoderheating, RSI are just a few I have dealt with personally that sell, build, fix, or install. Roger, willieG,jackyl440, and guru are a tiny sample of some dudes with terrific insight on these things. I know I left tons of good people unmentioned, forgive me. These are just a handfull that I have had some contact with that really steered me in the right direction.
When mine goes to boiler heaven I am leaning toward P&M. But will be contacting a bunch of these guys first before any check is written to see what is new and improved and within my budget.
Please remember, this is all my humble opinion. I am NOT an expert.