I've bought a bunch of IRF830 mosfets, but i think they are too much for the job. They are rated for 500V.
The voltage rating is the maximum rating and it's fine to use it in a lower voltage application. Just about any MOSFET is gong to be rated for more than 12V, so usually that's one spec you don't need to worry about.
The current rating is also a maximum. But, you usually need to heatsink the device if you are running near the maximum. And, you can burn-up the device with less current if you operate it in a linear (non-switching) mode by dissipating power, with current through the device and voltage across it at the same time. (Power = Voltage x Current).