I've bought two machines in that condition and both need a little work
- both needed a good cleaning and descaled... good to learn if you don't know how anyway.
- One needed steam valve seals and a steam wand, plus new group screens.
- both need group gaskets.
- one didn't come with portafilters... they can be expensive.
Once cleaned up both worked very well. Maybe I was lucky but a good cleaning fixed 80% of the problems.
Go look at it. Look under the hood and that will give you an idea of maintenance. If the blowoff had issues then there will be evidence of that. If it's super filthy inside you'll need to dis-assemble soak everything in citri or malic acid and re-assemble. You need to buy gaskets for heater, and some other places.
Ask where it was stored. If it could have frozen you could have a split boiler or copper tubing.
Check the prices on all of the above and it will give you an idea of what it will cost.
Also check pricing on the Pstat and computer... if the computer is shot that could cost $1000 or more.
It's always best if you can test drive it, but a thorough look will give you an idea of what shape it's in and how well it was cared for.
IMO Rebuilding one is good experience for maintenance.... if you plan to do that yourself..... but then again I like to tinker.
