Amp longevity, if well taken care of --the risks of purchasing used equip.

I don't know that we have even an approximate answer to that yet for class D amps in general.  Both the method of amplification and the power supply design are quite different from traditional SS amplifiers.  The current generation of class D amps for sale mostly use designs from the last 10 years or so.  ICEpower audio amplifier modules became available around 2005.

You might want to contact Bel Canto and ask what they would recommend for long term maintenance of your amps. 

35 years of buying used gear. Exclusively!

And still buying used gear occasionally. 

Time changes things sometimes. My main concerns with used equipment (solid state) are lateral mosfet outputs and with ARC potted "magic electronic component" components... The magic is you don't know what's in there, if it fails you're at the mercy of ARC. Idk if there is a solution for TO3 packaged lateral mosfet outputs as of this date, tons of exceptional equipment could be "bricked" if these fail. 

Most tube designs can be repaired with transformer replacement being the biggest concern.  

M 10 years old, well taken care of and not used all the time.
What kind of life might I expect from them/ worth repairing?

There’s a general and a specific answer.

Generally, one should expect amps to last 20+ years with average usage, tubes not included 

Specific, it mostly depends on how hard the individual amp was driven.

Assuming the seller is truthful, the 1500 amp should have no problem.

Whether to service later when needed is not so clear cut, but likely not worth the cost aka better to just replace.