Personally, I'd recommend getting hold of Silent Hunter 3.
In the first place it can be bought for very little, second it has been through many patches and can be tweeked to play without any problems, third, when you get into it, there are very many add-ons for it that can make it more challenging, fourth, there are a lot of 'help systems' options with it (such as crew helping you identify targets and working out firing solutions for you so you can effectively just point and fire with the periscope), which means it can be made very easy to play until you understand how everything works, at which point, you can gradually turn the help off and go for the level of realism you like. Lastly, being slightly older than brand new sub simulations, it runs well on a very modest machine.
On top of all that, there are literally thousands of posts on this forum about it, so a quick search on pretty much any topic you are having difficulty with can be investigated for a solution pretty much instantaneously. And if you cannot find an answer to your query, a quick question on the forums will usually get you an answer very quickly.
Hope this helps.
Chock