Drinking a lot of water indicates something may be wrong. Could you ask your vet to do a blood panel, if you haven’t already done so?
If you do get the water fountain, get a metal one. The plastic ones forms a “slime”.
They ALL will form a slime after awhile. It's a combination of their spit and microbes in the water. It should be cleaned weekly - which CAN be a pain. Look for ones with a charcoal filter that cuts down on the nasties being recirculated through the pump. The hard plastic kind is not hard at all to clean the slime off of and, as long as it is cleaned fully and fresh water replacing the old, it helps to make the whole clean up quicker. Glad to hear your kitteh is doing better.