Playable in the base game is limited to US subs from the 60s to the 80s. There's a DLC coming which is adding more playable subs and nations I believe.
The game is very easy to mod and there are some great mods out there that let you play pretty much anything you can think of from the modern sub catalog. I'm really enjoying playing a Swiftsure class HMS Sceptre that my Dad served on many years ago.
For difficulty - the game does look a bit intimidating but once you get a couple missions under your belt you'll be talking tactics like a pro
