Dave,
I am no expert as Ive only been doing DCC for about 2 years. But over that time, Ive installed around 20 decoders. With that being said...
I would say the biggest problems your going to run into, particularly in N gauge, is that of space and trying to find a good place to put a speaker and its enclosure so that you can get good sound out of the locomotive and decoder and still get the shell on it and have it run well and look good. This is, for me, and I think most guys would agree, the biggest challenge you routinely run into when doing a sound install.
Installing decoders is pretty straightforward on most modern/new releases. Some of the older locos out there arent 'dcc ready' and can be more of a challenge. Ive really enjoyed it. My advice would be start out with a simple install, then try a sound install, then try a hardwire (non dcc ready) install. Try adding backup lights, custom ditch lights, rear ditch lights when your skills improve.
I also learned a huge amount off of many of the incredible modelers on this forum. There is a vast amount of info on here and I find myself using the 'search' function alot. The guys on here are outstanding and always to give help, constructive criticism, and pointers whenever you like them.
I have the NCE PowerCab system and love it. Very easy. Everybody's got their favorite DCC system, and I really like NCE.
Have fun,
Brad