I'm looking into buying inexpensive rigid fork (~50 EUR) for my old GT Avalanche 3.0 (disc brakes) to make it slightly better commuter bike. My daily ride is 10km one way on asphalt surface. What should I consider when choosing a fork? What parts would I need in addition to the fork itself to do the replacement? Won't I make it much worse? I'm okey with what I have now, but would be nice to go little faster.
To convert to a rigid fork consider
1) Fork length - to maintain head angle geometry This depends on the travel of your existing suspension fork and the wheel size. ie For older 26inch wheels - an 80mm Suspension fork was a 420mm rigid fork and 100mm suspension fork 440mm etc.,
2) Brake mounts - whether your disc brakes are post-mount, IS Mount or flat mount. Adaptors can be bought to convert between them.
3) Disc rotor size - some rigid forks will have a maximum rotor size clearance.
4) Tyre clearance - though you are using it for commuting - bear in mind if you use an MTB tyre in the fork - determine the maximum tyre clearance.
5) Fork steerer length - if you are buying secondhand (especially). New forks will have an ample length of steerer to cut down. But secondhand forks, you must check and measure to make sure there is enough for your frame.
6) Fork steerer width / profile - unlikely to be anything other than a straight 1.125inch on your bike - but with tapered head-tubes you may have to change the lower crown race.
Answering secondary questions
What parts would I need in addition to the fork itself to do the replacement?
You should consider replacing the headset, or at the very least servicing the bearings if they are not too worn.
Won't I make it much worse?
I'm okey [sic] with what I have now, but would be nice to go little faster.
If you are riding on paved surfaces you will not really need suspension and you'll appreciate the reduction in mass. I don't think you'll mess up your bike (but do make sure you get the right length fork as a different length will alter your steering geometry).