If your able to drive, then my only suggestion is to arrange a hire car during your visit.
Due to the location of the Beacons, public transport is not ideal, with the nearest railway stations some way from the hills themselves, so if you do not have your own transport during your visit, it will require the use of expensive taxis.
As for airports, the nearest one is well over 50miles away, the other end of Cardiff, so its pointless even considering it.
The only real option, is to hire a car, and if you do not drive, then using public transport will not be easy.
The accommodation is very straight forward, as there are numerous Youth Hostels and B+Bs in and around the Brecon and Merthyr areas, but the only issues i can for see, is access to the hills.
It can be done, but your so reliant on the punctuality of the local buses, that its possible that you may miss the bus through no fault of your own.
If you can drive, hire a car, it will make your visit a lot easier, and you will see so much more.