First, you're actually better off with the track isolated into blocks, since it's good for debugging. Second, decoders are a big, big subject. Some locos, especially those with sound, come with decoders included. Some N locos are available with decoders but not sound. Non-decoder equipped N locos can be pretty difficult to install decoders if they don't have 8-pin NMRA plugs (especially non-plug split frame N locos). The best DCC system is a matter of personal preference, but I've found (as have many others) that the NCE Power Cab is good and worth investigating further.