Subsim is definitely the place to go. Make sure you update to the 1.4 patch (official patch). References to version 1.5 means the "U-Boat Missions" expansion pack, which is a separate purchase but requires SH 4. Many of the latest mods require you to be running version 1.5, but some older versions of the most popular mods like "Real Fleet Boat" and "Trigger Maru Overhauled" can be found that will work with 1.4.
Personally, I haven't installed any mods and am running stock 1.4 and have recently resumed a career that I started quite a while ago and I'm really enjoying it. Even in stock form I find the game very immersive, but you need a healthy dose of patience. This is NOT for high paced action junkies. Even with regular use of time compression expect to take a long time to complete war patrols.
Last edited by MetalMania; 03/21/13 06:30 PM.