I will admit I have installed plenty of used oil boilers, some fairly new and some quite old, but none of them give any bother.
All were basic non condensing boilers.
Massive difference between the cost of a brand new oil boiler and flue etc, and just the cost of a used boiler that might connect to existing flue, if it is okay.
If we can buy a second hand car and with a little maintainence, put it on the road, I see no problem installing a used oil boiler if it is to regs.
There isn't much difference from doing a full service/repair on an old oil boiler and installing one. You still have to stand over the boiler you worked on.