I had the same concerns as you, OP, about whether to get my elderly border collie’s teeth cleaned. She was a stray, and she may be anywhere from 14 to 17 years old. But I know - cause the family business I grew up in was a dental laboratory - how dangerous it is to allow infection to grow in the mouth. It can cause all kinds of scary things, including heart disease.
So I did the blood work prior and went for it.
Violet came through just fine and is a whole lot nicer to kiss now. 
I think you ought to do it. Can you do the Care Credit thing to help with the cost? Any chance the rescue would help you out? Most rescues will kick in for vet bills to fix a condition the dog had when he was in their care.
