With all respect to Mountain ash - there's no way I would have wanted to live somewhere like that when I was 18 (that said I also wouldn't have considered moving in with a girlfriend OR buying a house at that age either).
How about somewhere like the Graig in pontypridd - houses are still cheap, close to the train and the a470 for getting to places, and at least you have a bit more choice on shops/pubs/bars/restaurants