Do you now what the specs of the used computers are? Try to search for the estimated hashrates of your components and try to calculate your estimated earning. A good website to do this is: https://whattomine.com/
MinerGate is a GUI miner, try to use the console miners because they are more efficient. MinerGate is a good way to start. It was also my first miner but if you get more advanced, you should look for a pool to mine. Maybe try watching some videos on youtube about mining. (https://www.youtube.com/channel/UCjYHcWGAjUVqU49D2JOKD3w)
Good luck and if you have any questions, just ask them! ;)
RE: UPDATE new mining rig