Q....I know that straight sheet rope will not wear as well as wire BUT will it do the trick?
"Sheet rope?" Not certain what that is. Anyway... yes: Straight rope will work fine.
How much more expensive would spectra rope be and would it be worth considering? The boat is an old cruising boat so am not worried about performance just strength.
Then the answer, for you, is no. The purpose of high-tech ropes like Spectra and T-900 is to allow a smaller-diameter rope, with less stretch, to do the same or better job as a larger-diameter rope. Why is the smaller-diameter important? To reduce weight aloft. This is purely a racing thing, where every tenth-of-a-knot is critical.
In attaching the main shackle to the halyard [if it is sheet rope] would I be better whipping it or tying it on? I am concerned that if I tie it, it might mot allow the main to fully seat home at the top of the mast.
I tie mine on. (Poor splicing resulted in the shackle coming off [while the PO still had the boat].) The knot comes nowhere near the top of the mast. If yours does, perhaps a new main is in order? Besides: By the time you add a splice and the size of the shackle, it'd probably be about as much length as a small, well-tied bowline--if not more.
If you want to switch to all-rope, you may have to replace the sheave at the top of the mast. You won't be able to run rope through a sheave that's designed for wire.
We plan to replace our main halyard sometime in the not-too-far-distant future. Want to get a shackle on it, and I wouldn't be comfortable trying to splice the old line. Plus, the current line is quite stretchy. We race, but it's just club racing around the cans, so we have no plans to go with Spectra, T-900, etc. (Well, I
would, cuz I'm that way, but The Admiral and family CFO has nixed it
.) I plan to go with New England Ropes VPC as a good compromise between performance and cost, upon Maine Sail's recommendation
. It'll probably allow me to drop 1/16" and still have less stretch than the rubber-band that's currently rigged.