Help needed - buying a car that isn’t for sale...

Long story short, there’s an S15 Silvia that’s been sat on a drive for years, around 5 minutes away from my house. I’ve been looking at one for my next car for a while and it’s what I wanted, an AR2 red Spec R (like in the picture). I know it has been sat unloved for a long time cause it’s now turning green with moss etc.

I was wondering if anyone here had any advice for buying cars that aren’t for sale? Do I leave a note or am I better off walking away because I don’t know the history etc? I was thinking of giving a way lower offer because of the condition.

Thanks in advance!