Pressure testing an old hot water boiler; 1963 Bryant

This boiler was not started this year because of property changing hands. Electric heaters have been running in the building. It is a 47 bed nursing home....fair size building with 2 wings, that the town would like to acquire if things haven't been badly frozen.

All piping has been drained as possible, but not truly winterized.

I was thinking that for testing before spinning wheels cleaning that I would:

Disconnect the compression tank (It's in a sweet spot..see pictures)
Remove the pressure relief valve.
Fill the boiler with water up to the compression tank connection.
(There are no isolation valves and maybe no zone valves.)
Put air pressure to the entire system thru the PRV port which is located on the 2 1/2" copper supply line.
The boiler would be full of water and the air would be above it in the supply line and the return line.

Is this a viable idea? If so how much pressure would one apply to an old girl like this. The existing pressure gauge had the marker needle set at 15 PSI.

I don't want to put water pressure to the entire system as the piping is run in a sort of heated attic, under floor and other places unknown. I figure it is better to hear an air leak than wait for water to come thru a ceiling.

If it appears to hold pressure then clean the boiler, burners etc.

The nameplate says propane but it is and has been firing on NG.

Any advice greatly appreciated. There are 4 standing pilot lights on this. Any pointers on that system?

Thanks