What do i need to know about buying a new back wheel for my mountain bike
To answer the other part of your question, No you can upgrade the groupset later.

Assuming you buy a wheel with an 11 speed hub, then your existing 9 speed cassette should fit it, and an 11 speed cassette is the same width but with thinner chain.  You'll need a new chain of course, and a right-hand shifter that has 11 positions not 9.

You should be able to use the front chainring as-is, subject to wear.  

I can't say whether you will need a new rear derailleur or not - you should get away with the current one, but its not guaranteed.

New shift cable and housing too because why not?   



Since you've mentioned Tubeless in a comment, probably best to add new tyres and sealant.  Add a spare tube to your pack as well, for bad punctures. If your front wheel is not tubeless compatible, you're up for a new wheel there too unless you only want tubeless on the rear.