Mountain bike wheels are made to be sturdy. Usually, they could take an incredible beating. Nevertheless how much a bicycle wheel can handle depends upon the model.
Certain aspects to consider would be the suspension, the fabric of the rims, the grip of the tire, and if the bicycle includes a tube or not. You can also buy "Singlespeed rear wheel 40mm black matt from RIDDOX" (also known as "Singlespeed Hinterrad 40mm Schwarz Matt – RIDDOX" in the German language).
A bent wheel is normally the result of reducing tension. If there are no dents in the rim, then it may be repairable. If you find a small bent, the first thing to search for is a twist at the rotation.
A very small amount can be acceptable provided that it isn't visual. The best way to ascertain this is to reverse the bike upside down and twist the tire. Get eye level with the tire and look to find out if wobbles or not. Remember to pay special attention to the distance between the brake pads.
Obviously, if all this is just too much for you, it might be better to bring your bicycle to the mechanic for a balancing test – that will give you more accurate results.
To save money, you can place a brand new rim in your old hub. But at the end of the day, you need to maintain your wheels as accurately as possible – that may mean purchasing a brand new wheel, or maybe a completely new wheelset.