My wife has two implants, only wear one however when we go mtbing. Never had problems with the device falling off. Sweat also not a problem, she also plays netball and just wipe it dry afterwards or put it in the "dryer". Good luck with the operation, I believe it will make a big difference.
My single speed. Tried to convert as cheap as possible - spacers, 18t sprocket, tensioner and 8speed chain worked out ± R530. Used existing 32 middle chain ring. Enjoying every minute on the bike except the real steep stuff, still need to strengthen the legs a bit :-)
Don't worry, thorns not that big an issue. Being using these the Bottelary trails for years and haven't had that many punctures. Must have protection (slime) however and it also helps to keep your eyes open for the green plants on the edges of the paths! Go and enjoy. I must add that most of the riders that I see on the trails lately are not using the required permits :-(
