Re: Questions and Advice for that first BIG sail in a new(OLD) boat
It may sound obvious, but your existing running rigging is a good general template for replacements. When I replaced mine, I used masking tape to attach a 100' spooled measuring tape to the halyards just above the sail fitting and ran it up the mast, noting the length and used the measuring tape to retrieve it. Measure the run from the mast to any clutches and winches and add what you think will be a comfortable tail.
The existing main sheet and tackle can be removed from the traveler (usually with a cotter pin on the bail or the block) and measured on the dock.
The Genoa sheets will depend on the layout of your turning blocks, cars and winches, but the old ones should give you a good idea of the length. More is generally better than less and the type and "hand" of sheets can be important if you have to move a lot quickly. Likewise the spinnaker sheets will depend on the foot of the spinnaker and the "J" measurement ( distance from mast to Genoa tack at the base of the forestay) of the boat. I ran mine to turning blocks aft of the winches which increases the length required.
In all not an inexpensive undertaking.