—— 1) Do you need a wee remote mic? I’ve seen what looks like a little lapel mic, is that necessary if holding the go-pro at arms length? (I guess I’ll be able to figure that one out quite quickly by myself)——
If you want to remove wind noise, then yes, you’ll need a better mic solution. The Media Mod is very useful, but means the GoPro is not waterproof.
—— 2) How big an SD card for a long ride’s worth of footage? I’m guessing several GB minimum?——
Cards are cheap enough, just get the biggest you can.
—— 3) Are spare batteries necessary?——
Yes, they never last as long as you’d hope. Also, if you have spares, you can choose when to shoot/change batteries. (Not afraid of running out etc) I have about 5 spares, so I don’t need to bring chargers away on trips etc.