You are really buying two things when you buy a bike, these days. A frame and a set of components.

If you looked at most bikes, that are not expensive, you will find the same brands of components on the bikes regardless of the brand of the bike. Shimano components a plenty. If you were to look up an online bike catalog (for example bike nashbar), you would see the different levels of shimano components with the price increasing with perceived quality. Suntour is out of business, I think, and I do not know if any other manufacturer is in the lower end of the market. If you go to the upper end of the market, you can find more brands.

As far as frames, you want three things: fit, strength and light weight. Don't sacrifice the other two for weight, if you are in the lower end of the market. The material a bike's frame is made from can be steel, aluminum, carbon fiber, titanium, and probably a few other things. Buy cromoly steel, unless you intend to spend more money.

The other decision with a mountain bike is suspension or no suspension. The cheapest suspension systems are the front forks only.

You might be very well off to buy someone's frame which sat out in the rain a bit too much and learn to do you own work on the bike to strip it down and rebuild it. It takes 4 hours if you are not slow. And if you know how to do it, then youo are not going to get a teenager trying to fix your bike and doing ti wrong, whihc has happened to me.