I have read the replies to the O.P. with interest a lot of good advice from various posters , if I may I will add some of my thoughts while not claiming to have all the answers my thoughts on the subject come from being employed as a Highland stalker for much of my working life.
Clothing as others have said tweed is hard to better, while not waterproof it is water resistant ,plus 4s rather than plus 2s
or somewhere in between if you don't like them to long, a couple of reasons for this if you have to crawl when you stand up the wet area should no longer be on your knee but hanging somewhat below it a proper pair of plus 4s should be baggy enough to shed the water on to the ground and not down into your boots as tighter plus 2s are apt to do ,team them with a pair of gaiters and you should be more or less waterproof from the knees down.
Other advantages of tweed are that is not overly cold when wet unlike cotton which becomes very cold and uncomfortable when wet, also choose the pattern wisely and its as good as any cammo.
Highland stalkers have stuck with it for the best part of 150 years so it must have something going for it.?
Boots many favour high leg boots nothing wrong with them if that's your choice,personally I prefer a 4seasons ankle boot teamed with a pair of gaiters , a good 4 seasons boot is waterproof so no need for gortex, a good pair of ankle boots and gaiters are quite capable of wading a Highland burn without any water entering the boots.
Jackets choice is yours but a base layer that will wick away sweat is important, during stag season I would often wear a lightweight cammo jacket over a fleece in winter I like a smock again be careful some will cause you to sweat to much but you want something that's windproof in winter.
Sticks shooting sticks are a waste of time on the hill 99.9% of shots will be prone so bipod or whatever method you feel comfortable shooting prone, while shooting sticks are not much use on the hill a good stout walking stick is not only is it a third leg but is very useful if you need to drag, short rope tied centre of stick and use both hands behind your back to drag keeping the rope short means you lift the animals head of the ground making it much easier to drag ,using a stick is easier on the hands than just a rope, avoid any type of harness as they can be extremely dangerous on the hill if a beast takes of on a slope you will end up being dragged with it which could be disastrous.
Rifles any deer legal caliber is fine for Highland stalking , while some might be considered a bit mentioned southern or woodland stags this is not the case with our smaller Highland beasts .
Scopes again the choice is yours but there is no need for fancy tactical scopes for Highland stalking either a fixed scope or a variable whatever you choose just remember your shots are likely to be sub 200 yards ,most shots are somewhere between 100 and 150 yards with 200 being pretty much the maximum would certainly not let a client shoot at more than 200.
Zero someone mentioned holdover ,many Highland stalkers tend to use the inch high at a hundred yards method which allows them to shoot from zero to 200 yards without holdover.
Range finders again if you feel more comfortable with one that's fine, but judging 200 yards is not too difficult a little bit of practice and a range finder should not be necessary for hill stalking.
Moderators most people have one these days they are great for protecting your hearing as for actual stalking on the hill some claim they allow more chance of multiple shots at hinds on the hill, can't honestly say I have found them to make much difference there are still quite a few pro stalkers shooting unmoderated rifles.
Food and water I have already stated my views on carrying water on the hill so will say no more on that, food you require something easy to carry something you can stick in your pocket, a couple of Mars bars or snickers will provide energy ( not so good if you are diabetic ) though you can now get sugar free energy bars which are a good alternative, as is a banana but not a pretty sight when its been squashed in your pocket as is apt to happen lol