In terms of body protection, I personally prefer the following strategy:
-Full cavity wax treatment(Do not confuse with undersealing). This involves filling the body cavities with a thin oily product, it will penetrate the seams of the car and prevent moisture from entering and remaining where it can begin to corrode the metal. This is best undertaken with a quality product, some of my favourites these days are from the likes of rustbuster, dinitrol, bilt hamber(even waxoil is a half decent if old school product) This process should be repeated every 3-5 years as the product will drip/dry/run out, frequency depends on your driving habits/mileage.
-If the car is extremely good fettle underneath, I would recommend it is cleaned carefully, then inspected with a bright LED lamp for any small areas which have the beginnings of surface corrosion. If there are any, you can then treat these small areas with a wire wheel and rust killing solution, use some zinc based primer/red oxide to seal. Then a 'thin' top coat of quality underseal of your choice. I like to use a product which stays slightly tacky/oily for life, and again -goes on thin! Don't plaster everything in thick horrible goo, it can hide the beginnings of rust for years and make it worse.
At the end of the day nothing will stop a car rusting forever, you can only delay it by taking sensible steps.
Just my 2 pence! Everyone has got a different opinion. A lot of the true 'survivor' classics I have seen(Which have not been stored in a shed or come from a desert climate), have remained in such condition because they were cavity waxed from new by the dealer. A common treatment was 'ziebart' back in the day, look for that on classic car adverts kids!