I have been offered a decathlon mountain bike at a really good price. How easy would it be to convert this into a more commuter friendly spec (mudguards rack etc). Does anyone out there use a mountain bike for this purpose?
You need to check the chainring. This is because a crank drive kit would only fit a bike with at least 42T ring or larger.
Your RockRider ST900 may have the small 32T ring. If it's the case, you won't be able to fit the BBS01 or TSDZ2 kits. They will foul against the right chainstay.
If you go for a rear wheel conversion, you have have fewer choices in 17.5" rims.