Hi Bob
I highly recommend that you read this book
http://gb.trapletshop.com/an-introduction-to-radio-controlled-scale-sailing-models-by-phillip-vaughan-williams. At 9.99 it will give you all your answers.
Donor hulls, have a look at plans for the Louis Heloise
http://gb.trapletshop.com/louis-heloise which is a transomed fishing boat, or the Waverly Y2K hull. Or as you suggest you just could scale up another model. If so, try trawling the net for just the plans that someone may have left over.
Will it need a fin keel?
If less than about 800mm, then most probably it is essential.
Larger scale models can be ballasted, but may take up to 30 lbs of lead. Although you can stretch the hull vertically to give you more depth. Or attach a fin keel.
Your sail suggestion is practical.
Servos. Rudder correct.
Sails, there are lots of options. If you want to control the yards then you will need drum winch and pulleys, and a cats cradle of rigging. But this can be put below deck.
If you go for a rotating mast, then you can control the other fore and aft sails on a single sail ARM servo. I am controlling 4 sails on a single sail arm servo on my schooner.
Again, all of your questions are covered in the book, and at under a tenner, it is well worth the investment. There are other slightly more expensive books on the Traplet website, but to my mind not as good.