1. Usually at your height, M is recommended. Even if you find it a bit big, you can get a shorter stem to shorten the reach (distance between BB/saddle to handlebar).

2. Are you new to MTB? When I was new, I can't really tell the difference between SLX and XT. In any case, just go with the best spec that your budget can afford. SLX and XT now are really good. Can't go wrong with either. If you do get really deep into the MTB hobby, you will probably change to a new frame or get a new bike instead of just upgrading the groupset. lol.

I haven't tried the new deore but I've heard its good as well. Don't worry too much about it.

Don't worry about your first bike as well. No need to spend too much money on it. 99% of people that get hooked on MTB will end up upgrading/selling their bikes very quickly. You will discover that there is always something better and nicer to upgrade. lol.

Buy a new or used bike within your budget and wish list, and get used to it first on the road or parks. When you are ready, take it off road and see if its something you will like. You may end up liking it a lot or hating it so much. Then you can decide if MTB is the right kind of riding for you.

hybrid bike with rigid fork. Seldom bike shop will have ready stock for this type. Not sure is either low demand or less model be found in the Malaysia market. But anyhow you can try build yourself since that frame is hardtail MTB frame.
Hmm.. in market now still got 26" 27.5" 29". If you choose 26" 27.5" then that is not big different at all. But if you choose 29" then feeling is not same. M size in 29" and M size in 27.5"/ 26" you better go to try first.

now already 2021 years. not same like old time, you aim SLX XT groupset but at the end you only get is SLX / XT RD and the other parts like crankset / cassette / brake all are lower grade for complete bike. The manufacture very smart now days. Cut cost tons of it. Especially front single speed setting. You want XT , then put 1 XT in RD and use SLX on other component. Some more very geniuses pair it with MT-200 brake.

That also not important thing.

If you wish to ride bike in long term first you need decide either want go hard tail or soft tail. Then no need to waste more money in future. This is most significant in the first step you need to decide.

Deore already more than enough for your usage. Anyway first part which will got problem I believe is BB52. You need take care of that. Good luck have fun.

Nothing you can change regarding 2 pieces crankset even the most top grade model also same. If you are not often off road then you no need worry at all.

Most importantly is to get the right size.

Hop onto the bike and get the feel.

If can test ride lagi good.