A folding prop will greatly improve the performance of an inboard powered boat, but they're very expensive, and it's an expense that won't be recaptured when you sell an older boat.
I'm not sure, but believe an S2 with saildrive would most likely have come from the factory with a folding prop. They might have been optional, but would have been a very popular option on an S2, which were marketed as racer/cruisers.
The choice remains the same. How much are you willing to spend, in terms of boat performance or in terms of dollars, to have a swim platform. You might be better off to increase the amount that you're willing to spend on the boat, and look for a newer model that already has either a swim platform or a folding prop.
Generally, it's expensive to modify a used boat. You're usually much better off to buy a boat that is already equipped the way you want, and is better suited to your needs. You'll usually get a better boat, for less total cost, that you'll enjoy more.