Kachina cat

Kachina is not doing too well. I have been cleaning up her coughing up water/food for a couple of weeks kind of sporadically. I've also seen Cassi following her around and licking her butt like a mama cat--yuck! Well, this morning I found Kachina straining to poo, and she was walking around the house doing it. She was also vomiting.

I got her in to the vet and they are doing some labs for her constipation, etc. She is only 7 pounds now! That friggin hyperthyroid treatment hasn't helped her gain ANY weight at all. She was almost 12 pounds at her heaviest, so she's always been small, but I'm not sure if I am just delaying the inevitable at this point or I should jump through the hoops to find out what's wrong. Vet says probably GI related and after the labs come back, they might want to do Xrays, then if that doesn't help, an ultrasound, then a biopsy. It just seems like too much for a 17 year old cat, but where is the line? Now (labs) or the next step or the next one....?