Try/rent/borrow a few different kayaks before building, that should be helpful in deciding.

There is no one kayak that does everything well but a wider kayak does more things than a narrow one.

From what you have related my humble .02 is if you are a beginner and can only have one kayak that other beginners will use at all then let it be a stable one , the decision will never be regretted. You can always build a narrow one later if desired.

Keep safety foremost in your mind , wear that lifejacket and use good judgement about the conditions you will expose yourself to.