l would not buy a young bird older than 40 days, but to each his own.
vaccinations for ybs are paramyxo virus,paratyphoid and pox, some breeders do it some don't.
you can worm again, but you really should know if the bird was vaccinated, If you don't know I would wait and give them in 6 months.
as far as giving meds, only if the bird is sick and treat for that illness, otherwise you are medicating for no reason and that is not good, acv in the water and good feed and grit is all they should need, vitamins in the water or extra greens are good for them too., you don't want to forget about lice/mites, you could dust them just in case, but that is the only thing I would treat even if I did not see anything as it is topical.