Replacement of the lower part of a deck-stepped mast by a compression post is not a concern.
If the boat is older than '71, and has no placque, then, if it is an O'Day, it would be a DS1. At that vintage, the seats should be molded and glassed to the outer hull. I've seen fake wood plank patterns on the deck (not the seats) of very early DS1s (the ones with the wooden seats). The molded seats should show a faint diamond pattern (to make them less slippery). The same pattern should be found on the foredeck (molded).
A DS1 of that vintage would not have a CB cable, but a bronze handle.
So, there are some concerns that this may not actually be a DS1. Photos would help.
As to the wisdom of restoring a boat like this.
Financially, it makes no sense. Ever.
Buying sails isn't even part of that calculation. Sails are a
consumable - their lifetime is less than that of the boat, and sooner or later you will replace them. Same with much of the rigging. So, just add the price of that to the price of "hull-with-mast" and it begins to come close to the ballpark - that is, the lower range of (more or less) ready-to-sail boats.
The real issue is the unpaid sweat-equity. Now, if you need / want a winter project and your only alternative is to build a wooden boat from scratch in your kitchen, then this could be for you.
The laminate needs to be sound. No deflection, no soft spots. (There may be various wood reinforcements under the foredeck, side decks, in the transom - these are always suspect in a boat that age, but fixable - see under "sweat equity"). Likewise the state of the flotation. Just count on adding the inspection ports, removing whatever water-logged foam you find and replacing it with pool noodles (something people have done with more complete and pricier boats).
Second, the spars need to be sound (no corrosion) and straight, sail slots clear and not bent. (Masthead sheaves, if brittle and aged can be replaced)
With these three items sound, you'd have a foundation for a project.
A DS1 of that vintage should have had these pieces of wooden trim (in addition to "hidden structural wood"): floor boards, coamings on each side of the cockpit and across the rear. The coamings are required to complete the structural support. If missing you have no choice but to replace them - you decide how close to the original. There should also be two short pieces of wood (thwarts) that connect the CB top to the seats. These are also needed for structural support - if not present or not salvageable, they would need to be replaced). The wood used originally was mahogany for most of these items, so you may need to hunt for a supplier of suitable replacement wood.
If you replace the floorboards by a layer of foam core that you glass over, that would be a super upgrade, but technically challenging. It will make the boat "better than new" in terms of stiffness, but read the thread called "The core project" somewhere in the improvements section of this forum.
Making a tiller is not super difficult, neither is replacing the original rudder by your own re-build. Another item where you can reach "better than new". Tiller and rudder can also be purchased ready-made from third parties. Both in class-legal and in perfectly functional, but just not class-legal configurations. (Depends on whether you intend to race, or sell boat ever to a racer)
If you find that the CB is not in a good shape, then replacing it with one that has an actual foil shape to it will again yield something "better than new". Making your own, if you are handy, is not difficult, but third-party boards of good quality are available.
So, for some of the replacements/upgrades you may need to do, you can trade cash for sweat-equity. With sails, rigging and both foils you might get into an expense that would be close to what a better-maintained boat would cost you. However, you would have a boat that at that point, other than cosmetics, is already "better maintained" because it would have new sails, new rigging and high performance foils.
You still have to do the structural tasks of replacing flotation, inspecting/replacing structural and decorative wood items, so you will "lose" by not ever getting paid back for that. Now comes the cosmetic work.
If you are a stickler for a perfect paint job, you'll never break even
But if you value function above appearance, starting from a sound hull and spars should be at least somewhat viable project (and beats building your own wooden boat in your kitchen).
Much of what you read here might be useful, even if the boat you are looking at is not a DS, but let's see some photos.
Finally, if you are doubtful that you can get everything done in one winter season, done to the point where you can go sailing, don't buy.