well first disc CAN be used on the rear as well but only if you have the adapter on your frame alrdy if you dont then yes...only in the front.
Two if you want disc you have to do ur research on them and how much you have to spend...if you want great stopping power then go for some hydrollics but if you want something a little cheaper with still good stopping power with out the hastle of fluid then go for mechanical
three there are LOTs of disc brake makers out there, Avid is one well known fairly cheap company for example, and there known for there great mechanicals, Shimano also makes good brakes i have there deora disc brakes that work great just till they get dirty but they also have there XT and XTR line ups ect. and then theres the real hard core brakes that we wont get in to because im just gana guess you dont wana spend that much if your just now asking and getting disc.
My recomendation would be go for the cheapest mechanical disc you can find because your new hubs will set you back...standard price for cheap disc in the US is around $70 bucks if you want you can upgrade later once you deside you like the feel of disc, or upgrade to the good ones right away. if you have any more questions you should go to your local bike shop, they'll hook you up
~Tip~
you should find disc you are interested in online and talk to your Local bike shop about them and if they can order them for you if you still deside you like them, then they can set your bike up for you, but dont forget to look at hubs! , hope this helps
~Scott