Ive been coming here a few times a year for almost 20 years. How much cash to carry is always a personal issue. Me, even knowing my way around, I come with forints. Usually I have some from the prior trip, but this time, because I gave them to a friend, I had to go to BoA. Their exchange rate is reasonable. If you are flying in, the taxi will cost about 8.000 forints, They will take cards, euro and dollars, but Hungarian Cash is just easier and a bit cheaper. Then another 10.000 for the unexpected. This way I dont have to mess with anything from the plane to the apartment. Is it necessary? No, just easier for me.
Almost every place a tourist will go takes credit cards, but again, cash can be easier at times. I generally keep about 20.000 cash in my pocket for the occasional odd beer. A credit card for a 450 forint beer seems like a lot of trouble.
If you end up going home with 10.000 forints, consider it a souvenir. Oh, if you are flying out keep 10.000 for the taxi to the airport and a cup of coffee once there.