One of the most important things when you put a fork on a bike is how it "fits" the bike's frame geometry. Ideally fork and frame are designed together in what is called a "frameset". The geometry, which difectly determines fit and handling, is only complete when you consider a frameset, not a frame alone, neither a fork alone.
Nowadays, many mountainbikes are designed to be used with a suspension fork with relative large travels, for example 100mm. Then, if you take a rigid bike and put a suspension, you'll lift the bottom bracket, decrease the seat tube angle and decrease the head tube angle, unavoidably altering fit and handling.
Also, if you take a frame that's supposed to run a suspension and install a (shorter) fork, or even a shorter suspension, the same effects happen in the same way. That's why Surly (a manufacturer) produces "100mm corrected" framesets, which consist of a frame ready to receive a suspension fork, and a fork substantially longer than needed (about the same size of a 100mm suspension fork.
I have already made both changes (lifting and lowering the front-end of a bicycle via exchanging suspension or fork). The result in terms of rideability, when the bike is not "ready", is always a bit awkward.
Talking about fork materials and rigidity, from my experience rigid forks from any material are pretty rigid, to the point you cannot actually note any difference. Some steel forks have wide curves close to the dropouts, in order to absorb some shock, but that's a very old-fashioned design, and would look definitely "wrong" on a mountain bike. Mountain bikes should absorb shock via a suspension-fork, or wide tires, or both.
A side effect of some suspension forks is that handling becomes a bit wobbly either because of torsion/deformation of the fork itself while steering/cornering, either because of altered handling geometry (raising the front increases caster, making the bike potentially "slow" to steer).
I have seen broken forks, and have broken/bent some cheap ones myself in the past, but I think any decent one (in your price range) would be fine. Aluminum ones are probably lighter and very well suited to your intended use. You just should choose one that has at least a similar geometry to your current one, specially if you like the way your bike handles and don't want to spoil that.
Hope it helps.