Leveling the track

Dunno if this is helpful, but Bulldog had a video on how he levelled his track:
 
Get it all set up and aligned before you start sanding, IMO. It is a pain for sure, but it only gets worse from there, so prepare yourself.

As for level, I'd say it won't really matter when running on your track - your cars will just be faster or slower depending on what end the hill is on. But for sanding, I'd say its very important because you want all your sections mated up as precisely as possible. Will be very difficult to do that if you can't level it.