A common way to fix a fin is to have the fin extend into the hull - best is up to the deck level. This part of the fin sits in a box as a tight fit. The fin handles all of the power generated, a surface join is the surest way of losing the fin in the shortest possible time. Instead of a box, some have a pair of threaded rods coming out the top of the fin, which go through matching tubes inside the hull. For convenience, these are usually fixed by nuts at deck level.
The rudder will usually be on a rod which fits into a tube that goes up through the hull and has a tiller arm at the top. This gets a wire linkage to wherever the steering servo has been mounted.
A winch is good, but can be very tricky for a beginner. A arm type (look for 9Kg in the spec as a minimum pull) is much simpler to make reliable.
Fixing the deck is a matter of fixing inwhales (google is a help here) around the inside top of the hull to make a narrow shelf. I have used thin wood strips epoxied to the hull, and repeated until I thought the top surface was wide enough, then the deck beams can be fitted, and the deck skin glued over them.
Size and place of holes and arrangement of coamings and covers is up to you, but you need to be able to remove and refit the equipment you put in there through them.
3 pounds sounds about right - with real boats, if the wind is too strong, you shorten sail. On a model, you put smaller sails on.
Remember that all the power generated by the sail is transmitted via the mast and its mount, through the hull, to the fin, so that path needs to be strong enough. It also transmits through the line to the sail servo and its fitting to the hull, so this set of fittings needs to be strong enough. There is also considerable force going through the rudder and its mount.
At the same time, construction needs to be light, otherwise it will perform like a log. This is what makes sailboats so interesting.