I hose my bike after a muddy ride while the mud is still fresh and comes off easily, try not to have it pressurized though.

When I do a proper wash I use a bucket of soapy water, wet it with a hose first and also rinse the soapy water off with the hose after and then wipe it down well, lube the chain etc.

Job done most often is probably lubing and cleaning the chain - if you don't have already get a sram quicklink or something like that, means you can remove your chain easily for cleaning. Although since I started using Prolink lube I've found that I don't need to take the chain off. Wax based lubes are better in that way but you may need to lube it more often than if you use a wet lube like Finishline. You'll figure out what works best for you - but it will get dirtier than the road bike.

I use a camelback mule, since getting it would never go back to bottles. Fits all my emrgency spare typre & tools etc - and I take it even on 1hr rides. Main reasons is that it can be tricky to grab a bottle & drink while riding and yes also the fact that it can get dusty muddy etc.

Get yourself a good pair of glove if you don't have them already.

I don't do night riding so can't help there. Saddle height will depend on how technical your trails get - for steep dh types and lots of rocks etc you will find it's better to run it a bit lower than you would on the road bike (or in some cases alot lower) but yeah just experiment, it's the best way to figure it out.

That's about it - oh and for trails you'll probably want to run lower pressure in your tires, I usually run the lowest specified number on the tire sidewall (about 35psi) but depends on your weight too as to what you can get away with. Lower pressure equals better grip, higher pressure probably gives better rolling resistance, so depends on your trials too.