I'll be honest with you, this would be difficult deciding either way. On one hand, its hard to turn down a Lincoln 250a class MIG for $550, especially considering a comparable MIG new is $2k. I searched and searched for a decent deal on a Lincoln Power MIG 255, and took me the better part of a year or so before I finally found a niced used model for $1k. Most were going for $1200-1500, so I jumped on mine. On the other hand, these touch panel Lincolns make me a little nervous, and as you have found out, they are not cheap to replace. Plus, as you have also learned, the cap banks can be an issue. It comes with a new liner, so you may have to replace the old one before you can even try a weld. Add to the fact that it looks like you are also going to have to provide your own bottle of shield gas and regulator, as none is pictured.
Looking at the pics, its clear the machine was used quite a bit. One thing that would concern me, it doesn't have any wire and has been in storage. The first question I'd ask, is the guy the original owner or did he somehow acquire the machine? Next, I would want to know if he had any wire available, and if you would be able to fully test the machine. Does he have 220v available to test the machine? Of course, if he doesn't have any wire, that means you'll need to bring your own. If he doesn't have a way for you to test the machine, I would walk away.
As far as testing it, I would want to make sure every button on the panel worked without hitch. I would want to burn a good deal of wire through the machine, on various volt and wire speed settings, to make sure the arc is smooth and there is no hesitation. Make sure the wire feed consistent on all WS settings, with no hesitation there as well. Does it have the necessary rollers? Does the cooling fan function correctly?
There's a lot to check on these machines, and it means more than just running a quick bead. It would probably take me 30+ minutes to check out the machine before I dropped a dime on it, as there is a lot to go through. If the guy seems hesitant about any of it, walk away. Good luck!