Go for it just jump in, go for it. I have been reading your posts for a few years now and you really want a Fintail. The car looks decent for 43 years old!
I would just expect to put some money into it and wouldn't sell your SD for a few months. Then use the money from your SD to fix all of the rust the proper way. Than keep her clean and out of the salt and it will never come back.
What have you got to lose? If you get burned it could be worse. If losing $1,400 on a Merc is the worst financial move you make your in good shape!