I think that the idea that that salt water will wreck it after a few trips is completely wrong.
Remember these engines were designed for salt water use. The very fact that so many have survived is testament to that. They were not like cars of the 50s, 60 and 70s which rusted through helped by the occasional dose of winter salt on the roads.
I only use my FV/FVPs in salt water and often for a week or two at a time. They are either on the dinghy transom or on my boat sailing somewhere, and without a fresh water flush during that time. I have detected no corrosion problems whatsoever and am not the slightest bit concerned about them being used in salt water.
There are a few simple things which we can cheaply and easily do, and which will ensure that your FV/FVP will last another 2 or 3 generations of intermittent salt water use. By intermittent, I mean being on a transom most weekends in the summer, as opposed to be left permanently on a boat.
1. As I have already pointed out, the steel plates with their sealant on both sides are there to prevent the cooling water coming into contact with the aluminium head and crankcase. This is absolutely vital.
2. A fresh water flush is critical after use and before storage. A wash down with of the outboard should be carried out then with application of a dewatering fluid ie WD40 and a wipe down with an oily rag.
3. Apply some dewatering fluid or and/or oil into the block via the cooling water outlet. You can see from the two lower studs on your FVP that they have probably remained wet for ages, as they are almost in contact with the bottom of the wet block when stored. Rust preventing fluid is needed there.
4. I apply a zinc marine based white grease to certain parts eg the drive shaft tube and various other parts after flushing. This provides excellent protection in storage and when next used.
5. The FV gearbox should be drained and filled up with SAE140 directly after the engine has been flushed. Contrary to some advice given on this site, the box MUST be completely filled right up with oil when the engine is laid horizontally with the drain plug upwards. Screwing the drain plug in when it is brim full of oil will force the oil up the probably worn bevel pinion bearing, displacing any water there.
6. Good quality paint followed by grease needs to be applied to the drive shaft and the drive shaft tube. All bolts and screws should be assembled with grease.
Doing this and using the outboard, is the best way for you to enjoy it, and to ensure that it survives for years to come.
Go for it!