If you do decide to keep it order some H&K Push pins from rrages it will make taking the lower off a lot easier, you'll have to email him first before you place an order online as he normally doesn't ship to Canada, but he'll ship the pins without any problems.
That's unfortunate that the gun was miss-represented in such a large way, hopefully you got it for cheap enough that even with repairs it still comes out cheaper than a new one. If it survived a year in his trunk without every been cleaned, it means he didn't shoot it a lot or he stopped shooting it when it got dirty....
Hopefully you'll get yours up and running, they are a lot of fun to shoot. Also can you post a picture of the front part of the stock where the hinge is and release button, I'm curious to see what shape that's in too.
Thanks