You have a difficult situation all us pet owners experience - my sympathy.
I don't know what the side effects of the medication is, and what does it accomplish - reduction in seizure, symptom only?
My understanding is most small dogs have a life expectancy in the 15+ year range, so if yoiu can at least give the dog some quality of life with meds it seems worth a try.
As for the additional tests, what can that help? If they confirm a tumor, what that that point to? Surgery? If additional tests are not likely to point the way to a reasonable (cost/pain/risk) cure, then I say don't worry or "feel bad" about not paying for more tests.
Wishing you and your poddle more years of enjoyment together.