I expert HVAC guys. I need help with a opportunity to buy some used equipment and wanted to get your opinions.

I currently have 2 boilers:

Burnham boiler with a carlin burner. (mid 80's maybe?)
Naitional U.S. Sunray IV with Beckett burner(60's, huge cast iron beast)

The service guys all seem to believe the older one will last longer because the boiler is old school and built heavy duty.

My friend has a Weil Mcclain gold series energy star unit, not sure what burner is on it, and it has a hot water coil for tap water or whatever. They are both approx 6 years old. They are changing to natural gas so the oil stuff will be taken by the company doing the changeover and scrapped.
I'm wondering what you guys think of paying between $300-500 for the newer stuff and warehousing it in the basement for when 1 of the others dies. The ones I have are running 83% efficiency and the new one is doing 85%. Is it unwise to store these things after they've been used or would you suggest buying it and replacing one of the two immediately. Alternatively, I could just pass on it altogether.

I just don't think I can spring $5K for a new system should 1 of the 2 I have go bad. I have 2 because we have an apartment on their own heat/hot water. I'm more of a: if it ain't broke don't fix it so I don't really want to change one out just for the sake of it.

Anecdotally, my neighbor had the same burnham boil as mine and he recently had to replace it.