Giles,
Congrats on the find. As for what steps you should take when it arrives, it depends. First, look inside the boiler to see if it needs to be descaled. This, IMO, is the first place to look to see if you need to consider tearing it down right from the start. If it looks okay, look at the portafilter/grouphead gasket. If it's hard, there is a good chance it could benefit from an overhaul. If both of these look okay, fill the boiler 3/4 full with filtered water and turn the machine on the ensure it heats up to full operating temp (10 or so minutes). This will check the heating element and pressurestat. If they work, you are good to at least give it a shot at a few pulls to see if the seals/gaskets need replacing. If you see leaks anywhere, turn the machine off and unplug it. Find the source of the leak(s) and start shopping for replacement seals/gaskets (see link below).
Now, to prep you ahead of time for what all is involved in rebuilding the entire machine from scratch, visit
OrphanEspresso's site. This is an extensively detailed page on step by step instructions for rebuilding your machine. Doug and Barb will most likely have any part you'll need to get the machine working as good as new, and they ship worldwide. If you need anything, email them by using the "Contact Us" tab on their site.
Good luck, and let us know what you find.