The R5S is definitely fast enough for your current ISP service (~50% faster than the R2S, due to its higher clock rate more than its A55 versus A53 cores in the R2S), but I would still get the R4S, because the R4S is ~50% faster than an R5S, and the R5S is not supported by OpenWrt. FriendlyWrt is reported to have security issues.
Yes.
FriendlyElec's prices are quite reasonable for the power adapter and cable they offer, but I can tell you from experience that the 20W adapter from Friendly Elec is a bit large to play nice with another device plugged into an adjacent power strip plug. If that could be an issue for you, you may want to shop around for a smaller power adapter as long as you do not plan to plug things into both USB3 ports that need power.
For the R4S, figure on ~5W per USB3 port (it has two) and ~7W for the CPU - so it makes sense that FriendlyElec provides a 20W power adapter. But since I plug nothing into the USB3 ports on mine, I dropped back to a smaller 15W power adapter.