I know this car you are talking about, or should I say, I seen this car on Craigslist. It's the green 2 door for sale in Alberta with Saskatchewan plates correct?
If it is, I did the quick math and he is asking a little too much considering the work that needs to be done. I personally would be looking south of the border for a cleaner more complete driver for around the same money. I do live only 15 mins. from the Washington state border, so this may be an easier option for me. I have in the past seen some 510's for sale in Alberta that were actually pretty decent cars for not a ton of money. It does always does seem to be when you are looking, nothing is available. Take your time and look everyday on CL Kijiji, or whatever you use. Something will always show up.
Good luck on your 510 adventure and welcome to Ratsun!