Bicycle buying suggestions please!
Product recommendations are off-topic on this site because they are geography-specific, taste-specific and quickly go out of date. However, there are several questions in your question that can be answered (and as such, they should have been submitted as individual questions to the web site in the first place). Let me address a few of them.

The need is reliability, longevity and cheap maintenance.

In these terms, nothing beats a single speed bike actually. There are fewer details to wear and to damage on it. A bike with internal gear hub comes next.

I'm not sure, but the far I know, they just get the wheel coming to a rather sudden stop,

Brakes in general are meant to stop a bicycle, and the faster they can do it, the better. It is up to their operator to control them properly; after all, you can fall off your bike even without brakes, being perfectly still. Any type of brake requires proper rider technique. There are a lot of materials about proper braking on the Net.

and are also harder and more expensive to maintain. Aren't they?

No, not really. Mechanical disc brakes are as complex in maintenance as mechanical rims brakes. Hydraulic disc brakes are even simpler in everyday's use, until you need to bleed them, and then it becomes somewhat tricky.

What I also want to ask is, say, I get the Munich itself, is it a good idea to swap components before I begin?

No, it is not a good application of your budget. New aftermarket components are more expensive to get than if they came with a complete bike (bike manufacturers get a huge discount on bulk purchases, compared to us mere mortals)

Different websites told me that TX series is better than TZ series, hence the question.

Both of these series are pretty much budget components, to be honest. It is very unlikely you will feel a difference between then. Instead, learn how to tune your existing derailleurs in order to get the best value out of them.

I'm not sure if I should be buying online either

Going with a second hand higher-tier bicycle is also an option; it may be more comfortable to use because of the quality of frame/components to start with. Unless you are 100% sure about the size of a bike you need, you should not buy online simply because you might get a too big or too small a frame.

I've read various reviews that talk about how much more they've had to spend because of multiple problems with the bike (broken rims, problems with the cassette, derailleur, etc).

These problems are not specific to the online/offline markets. It is up to the people who assembled the bicycle in the first place, and the customer service they provide. Make sure you buy at a reputable place in the first place, regardless of whether it is on the Net or offline.

I'm also unsure whether triple chain rings are a good idea and would like to know if I should swap them out for something else or if I should just get a bike that's got a single chainring.

Triple chainrings are usable; people have used such setups for decades. At the budget limit you have, it is unlikely you will find any new bike with 1× setup; a used bike, quite possible. 1× setups have many advantages: weight, simplicity and reliability, fewer components related to shifting. Their disadvantages are: price of new setup and replacement parts, single point of failure upfront, &quot;sexier&quot; look and as such proneness to being stolen.