I am so sorry your cat is so ill.
Its a shame that her previous owner neglected her so badly.May God bless you for takng her in.
There are many causes for a bloated belly and some are serious.
FIP : http://www.peteducation.com/article.cfm?cls=1&cat=1316&articleid=212
Constipation, parasites, these diseases:
Try some human milk to releave constipation.
There are a few possibilities for these symptoms. I will try to cover everything.
Weight loss is usually related to hyperthyroid. The cats eats great but the excess thyroid hormone burns up all the calories.
The problem is that this disorder can lead to your baby developing diabetes, high blood pressure and other ailments.
We do have excellent meds for this but is is necessary to do a T4 thyroid blood test.Any other thyroid test can be inaccurate.
It can also be related to renal issues especially if there is excess shedding because the hair becomes very dry.
Stress weakens the immune system and can lead to illness.Dental disease also depresses immunity and sends bacteria into the organs.
You are absolutely correct to be concerned, dehydration is very dangerous.Sometimes it is caused by renal issues that begin with UTI's
Not only can infections damage your baby's urinary tract but her immune system as well.
The type of food is VERY important. Hi protein and MEAT puts weight on without unnecessary fillers.But protein is contrainicated if the cat has renal disease.
In addition, UTI's must be treated with an effective broad spectrum antibiotic like Clavamox or clidimycin and for a long enough duration so all bacteria is eradicated. If not, they will refluorish, stronger and more resistant and reinfect her . In addition, the bacteria can become tolerant of the same antibiotic so the medication should be varied for best results.
It is better if she does not eat dry food. Canned will ensure she gets adequate fluid. Cats her age can become dehydrated more easily. A dehydrated cat is not motivted to eat.
She should also be encouraged to drink(water will not balance her electrolytesI as much as possible to keep the kidneys and bladder flushed. You can give her one daily drop of unsweetened (only) cranberry juice. That will reduce the PH of her urine and make her bladder inhospitable to UTI bacteria.
She should be seen by a vet for some senior bloodwork so you can identify and treat her condition.
Please let me know how she is doing. I will always be here to help.