I want to by mtb bicycle, need your help.?

I can't tell which currency you're using, so I have no idea what 17000 would buy you.

15-21 gears, that puts you in freewheel range. Freewheels are an older, weaker design. For 15 (3x5) gears, they're pretty much OK, but a 21 (3x7) freewheel bike is quite at risk of axle breaking/bending.

Freehub designs, which start at 7, but USUALLY is 8 speeds and more on the rear, are stronger.

200-300 km is quite a lot to cover on a MTB. I don't think I'd be able to do that off road in a day.

If it's on roads, I wouldn't want to use a MTB for it.

And asking for a bike that's strong enough to do stunts, yet light and nimble enough to do 200-300 km in a stretch is really a tough call. Might as well ask for a sports car with good freight capacity.

Strong spokes is fairly pointless to ask for. Pretty much all stainless spokes will take about 3 times as much force as anything but a stick through the wheel can create. The most common wheel killer is something called fatigue, caused by a poor build. Galvanized spokes is another thing, they are weaker than stainless. Still entirely possible to build a perfectly serviceable wheel with galvanized spokes.

Never seen a bike where the saddle couldn't be adjusted.

Handlebars is a bit more so-so.

Older bikes used threaded forks and quill stems, which were easily height adjustable.

Newer bikes tend to use threadless forks, another type of headset and another type of stem.

Nowadays height adjustment is an entirely different thing.

There are usually a couple of spacers that can be moved around, the stem can be flipped or replaced.

The new design is a bit stronger/stiffer, but less flexible.

If you really want lots of adjustability on a threadless stem bicycle you have to make sure you get one with an "uncut steerer".

Pro quality mountain bikes have 30 gears. Bikes with 15 or even 21 gears are low quality. You wouldn't want to use a mountain bike for a 200-300 km ride you would want a road bike. A bike suitable for long rides like that isn't suitable for doing stunts

Keep $AVING. A "pro" mountain bike won't have 15 to 21 speeds. It'll have at least 24. 3 X 8 = 24.

A pro mountain bike will run 10 times that much you want at least 24 speeds.

What John M and O Hippie say. You also want brakes, brakes, brakes - you do not want anything that breaks.