It depends what you want to do with it! If you want to pull passengers, or just run it in public, you'll need insurance and boiler certificates. You really ought to be a member of a recognised ME Society to do that. This will give you access to club boiler inspectors, who should be able to give their informed opinion on the loco.

Even as a project for you to complete/ improve you will need at the very least a sound boiler . Ask ISRS to give it a hydraulic test . If they wont I would steer clear

Brian

which loco is it?

They have a boiler that leaks like a sieve from joints and another one that leaks from all the fittings and yet another that tests OK for nearly 3 grand but thin gauge and "the fire box has taken a set". Whatever that means

Andy,

Buy which one floats your boat – AND which one you can get still castings and drawings for but be prepared for trouble.

Hydraulic test to 1.5 X working pressure and steam test at working pressure. It should have had a shell test when new and it might be worth asking if it is a professional boiler.

Ask them for sale or return should the boiler fail, a new boiler will cost around the £2k mark.