Having gone through the rust thing with a 66' 912 and a more recently a 71 914; I know how you feel
There are quite a few good treatments out there. What you must decide is do you want to stop the rust you see or do you want to go all out and eradicate any potential rust.
To go all out will require a lot of stripping and work that is best done in conjunction with a full restoration. If you just want to do a good "patch job" then I would recommend POR15 or compatible product. These treatments work very well if used correctly "i.e. directions are followed" and do not require the level of prep as power coating or galvanizing (both work great if you have the car stripped down to nothing)
In the end just pick a product that has worked for others and fits with the magnitude of prevention you are looking for!