Are there any documents with it such as boiler certificates? Having such provenance will make things a lot easier if you want to run it in public. If you run it in public you will need insurance; the only really practical way of getting that is to be a member of a recognised Model Engineering Society. The advantage of that is that you will also be able to tap into expertise and assistance in getting it running.
However, if you're only going to run it in your back garden by yourself, then it's a bit simpler. For your own peace of mind before lighting the fire in it I would advise you to do a hydraulic test on the boiler. This can be done by means of the tender hand pump, assuming there is one. Do not rely on the pressure gauge on the boiler – how do you know it isn't wrecked because it froze up in storage? You should check it against a gauge of known accuracy before you trust it. Cue membership of an ME Society – they have such things.
The reason we test boilers hydraulically is because if it does go bang, you might get a little wet. If you use air or steam and it goes bang, you may need some wings and a harp.
Having satisfied yourself that the boiler will hold pressure, the simplest way of seeing if it's likely to run is to couple it up to an air compressor and see if the wheels go round. No air compressor? What about a type pump – but you'll need some means of connecting either device to the engine. Cue need for a lathe or membership of an ME Society.
Getting it in steam will require an electric "blower" (i.e. sucker!) which draws the air through the fire until the loco's own steam blower can operate. Start the fire with paraffin-soaked charcoal, adding this until you start to see steam and then switch to coal. Charcoal is good enough to try it out if you don't have any coal, but the loco is designed to burn coal, so you won't realise its full potential on charcoal. ME Societies usually have coal available.
Running it jacked up (stationary) with the wheels lifted off terra firma is OK for a moment or two, but it's far from ideal as the motion is not in its normal position and there may be unexpected bangings and such.
You mention "sourcing parts"; well there you may have a problem, because unless you want something like an injector or a simple boiler fitting, you're likely to be out of luck. Macc Models do a range of fittings, as do Blackgates, Reeves 2000 and others who advertise in the ME press. You certainly won't find a ready-to-fit left hand bogie wheel or a cab footstep in a shop, like you might obtain Hornby spares!
Then where are you going to run it? have you got a track in your garden? Cue membership of an ME Society with one.
So no – Don't don't – if you're serious about running it, go and find a local club, join it and seek advice there.