You only need a specific wheel / freehub body if it is a road cassette.

An 11 speed Shimano MTB cassette will fit straight onto your MTB wheel which is running the 9 speed cassette.
Obviously you’ll need a new 11 speed shifter, rear mech and chain too.

I’m fairly sure the M435 rear hub has a 10speed compatible freehub if it’s this one:

http://www.chainreactioncycles.com/shimano-m435-rear-hub/rp-prod159691

and therefore an 11speed MTB cassette will fit it.

The width between the lockring and outer edge of the alloy carrier is the same between 10spd and 11spd SLX/XT/XTR cassettes.

My 2009/10 Hope Pro 2 rear hubs runs perfectly on XT8000.

M7000 drivetrain groupset will be a mere £250 or less.

M8000 will be more like £350 but worth the extra cost, e.g. you’ll not ‘need’ to upgrade any of it, just replace bits as they wear.

If you’re going 1×11, use an online gear calculator to match your current first gear ratio, for example, 24×36 is about the same as 32×42. Or if you want a good place to start, 32T up front, 11-42T at the back. Oh, and make sure you get a Medium Cage rear derailleur and not Long (SGS).

I imagined he means it in terms of improving chain retention. A shorter cage means less chain flopping around over rough ground that might mean the difference between keeping and dropping it.

You also just don’t need the long cage as you don’t have to accommodate the big ring on the front any more. Less weight, less chance of snagging it on terrain and less chain rattle.