The Budapest Ferenc Liszt International Airport (BUD, http://bud.hu/english) can be reached easily from all cities using normal or low-cost carriers.
The city center, including Hotel Mercure Budapest Buda, is accessible by taxi, minibus, or public transport.
Taxi (provided by Főtaxi, about 22 EUR, one way):
Minibus (provided by airportshuttle.hu, about 11 EUR per person, one way):
International trains usually arrive at Keleti railway terminal: take metro M2 to Déli railway terminal, the Hotel is within 100 metres. If coming from the west or south you can get off at Kelenföld station and take a train to Déli.
Please look at the google map here:
The currency of Hungary is the forint (HUF), 1 EUR is about 300 HUF.
You can usually pay with credit card, and often euros are also accepted.