Welcome to the fleet MartinP.
I'll try to answer your questions and I'm sure others will be along shortly to give their thoughts.
1. In order to answer your first question, it would certainly help to know what your system specs are.
2/3/4. Directly above, you'll see a "Downloads" tab on the forum menu bar. Click on the SHIV category (after clicking DLs) and you'll find a veritable treasure trove of diownloads. Most of these DLs only work with version 1.5 so, if you have the wherewithal, it might be in your best interest to get version 1.5 (the U-Boat expansion pack). I don't know if any of them are for version 1.4 anymore but there might be a few. Mods are strictly a matter of preference so it's best to try them all and then decide for yourself. I'm currently playing TMO 2.0 along with RSRDC and love it, but I like RFB too. Both mods are more difficult (in different ways) than the stock version.
5. Stock (I think) gives you more to shoot at than all the mods. Ships seem to spawn anywhere and everywhere, sometimes the same ships over and over. In reality, many subs had to endure weeks of inaction or sometimes returned to base w/o any contacts. All the major mods as well as RSRDC address/correct this in order to balance what it was like in reality.
Good luck mate.
