Cheers Mate and welcome to this forum!
The right diffuculty level ist a difficult question!
Please note that difficulty level mostly affects swordfighting and crew morale I think. As far I know, it does not affect difficulty of naval fighting.
With swordfighting it is that the whole system has been reworked and now is a lot different from what it was in the stock game.
Basically, it is a lot more deadly. This is because you can't hide forever behind your block as you could in stock game. Enemies will by chance strike through your block. So, there is no single combat against a dozen enemies. Two or three *will* kill you, and quickly.
You can still block, but keep in mind that it won't work guaranteed. so you need to fihgt more offensively, and kill them before they kill you.
There are several ways to help you.
Good officers are one option, but you can take up to 8 crewmen to join you on land. This can be done by talking to them inside the crews quarters in your ship. The officers that join you on land shall be combat-trained (master of arms or boatswains are usually best) and shall have a high melee skill.
Crew can be equipped by puting stuff - blades, guns, armors - inside your weaponslocker.
Nonetheless, since you will be unfamiliar with this new combat, I advise to try one of the two easiest levels first. The decision can be changed everytime in the options, you don't need to start a new game for that. At the beginning, it is higly recommended to avoid combat ... because it is deadly when you are not well prepared.
About the overall - means: non-swordfightung - diffficulty:
Take a close look to the Options - Game preferences. There you'll find a lot more options that may affect difficulty.
At sea, you may often face largely superior enemies. But you don`t need to fight them all, in many cases you may better retreat. You may run into a ship of the line with your lugger, but well, you can outsail it.
Generally, you'll find the modded game a lot more realistic, more challenging, but alos more difficult than the stock game.
Have fun with it, and when something is not clear, feel free to ask.
