Apologies if this has been covered elsewhere, but I have been unable to find a review that states what if anything is locked for use in the Multiplayer until its ulocked in Career. 
 
Basically I only care about split screen multiplayer, so want to know if any of the tracks, cars, weather effects, karts, create a track etc is unavailable out the box.

I've not played a lot of arcade, but from what I have unlocked while playing A-Spec/B-Spec some tracks get unlocked and that is about it. I don't know if your A-Spec level effects what cars are available. But everything else is accessible.