Free boat for me - lots of questions
As Paul suggests, a lot of the answer depends on the particular model and what you expect to do with the boat. You should be able to get by with a 4 hp on a more modern design and will want more like 6hp on an older boat.
Whether it is worth fixing up the boat also depends on the model and condition and how fussy you are. To quote my oft mentioned old boat litany you can expect to find some ''issues'' with any older boat. Unless very well maintained and updated by a previous owner, you might expect to need to address some combination of the following items:
∑ Sails, chainplates, mast step and associated suporting structure, standing and running rigging that are beyond their useful lifespan,
∑ an engine that is in need of rebuild or replacement,
∑ worn out or out of date deck, galley, and head hardware,
∑ worn out upholstery,
∑ Out of date safety gear
∑ electronics that are non operational, or in need of updating,
∑ electrical and plumbing systems that need repairs, upgrades to modern standards or replacement.
∑ Blister, fatigue, rudder, hull deck joint or deck coring problems
∑ Keel bolt replacement (bolt on keel) or delamination of the hull from the ballast for a glassed in keel.
∑ And perhaps a whole range of aesthetic issues.
On a good solid design, you can easily get your money out of the boat but there are boats so ignoble that they are not worth yoru time and effort. When you know more post it and with that we should be able to give more meaningful advice.