Any impacts if any you may experience whilst foilboarding will virtually always be on the board rail. In 4 years of foilboarding
I have never had an impact with wings or mast. Usually impacts are shin to board rail whilst attempting downwind jibes, stuffing it up and getting dragged over the board. I only do upwind ducktacks so never experienced the previous as the board is always out of harms way in the event of a stuff up. Regardless, if you are unlucky enough to have an impact no wetsuit will have any protective qualities, unless it is 50mm thick.

I'd say legs. Unlike previous posters I've got a dark spot on one Shin where a mast impact took it down to bone.

I was foiling in the ocean today as it blew up to 25kts, I had 2 stacks where I landed in front of the foil and hoped it didn't hit me. One of them did impact and left a nice bruise on my lower leg.

Having said that I'd probably buy whatever you feel comfortable wearing.

I guess this is the advantage of lightweight gear, once you fall off there is little momentum and any impacts are relatively minor. I know that my heavier alloy foil could travel a lot further on its own and hurt if it hit you.

As above...
Protection - never enough or wear for comfort / practical..
Me = comfort - I always wear a shortie... except winter (= not much now!!)
The slice across front of wet suit from rear wing I think when I started foiling .. I was surprised to see it there as had no recollection of a problem.. 2mm material - not just arms and legs!!
My suggestion.. wear helmet - I was anal about this - fearful of getting hit by board or wing - this has not happened to me -- But the added profile of the helmet did cause more water impact concussion (90% of my crashes were head first sling / splats) - I had a rule 30 (crashes!!) at 30 (kmph - but probably anything over 20 kmph) = feeling a bit punch drunk if I continued. I moved to using sports head gear which has a lower profile fits better and protects ears more.. Once you are over that first stage learning hump high impact crashes tend to drop off rapidly.
I always kept my feet away from under the board - the foil bits are all solid and often sharp - so possibly consider booties to protect ankles till you get the hang of it..


You can always cop a scrape from some random aspect - that's life
Cheers
AP