If you look in the mods section there is a sticky for Grey Wolves with all the info you need and one for NYGM tonnage war too
Other major mods include RuB ( real uboat) and Uboat.war ace standalone 1.67
Have a look thru the mods page and see what you think
I personally prefer GW although have Uboat.war ace installed too on another SH2 installation- apart from the eye and ear candy parts to GW the map has a lot more ports and there are changes to deckgun strength to "up" realism - well too much for me to mention just pop over to the mods section and read the GW sticky and the others and make your mind up from there
http://u-boot.realsimulation.com/
Is a good download site with a section on all the major mods too
As for targeting at night that is the best time to sink merchants as you can get closer on the surface before detection and even closer underwater
Yes it is dark but you should still be able to lock on to a target with the periscope and UZO no problem
No different to day time really just better for you - less detection