I cannot reccomend using a Kress router for cutting metals - in the end it's still a router not a spindle. Whilst clearly from the video you linked to it is possible to cut metals with it, that's not really saying much on it's own. You could cut what they did in the video with a hand file, but that doesn't mean it's a sensible method. One thing you don't get from videos is the impact on the life of the tool, which is important here since that is putting a high force onto the Kress' relatively small bearings, which will have a detrimental effect on their life. One often sees videos where people are pushing the machine or tool too far, maybe it works, but if the video lasts 2 minutes and the tool only lasts 5 minutes at that speed then the video is misleading.

Much better would be to get one of the water cooled 2.2kW spindles from China that you've no doubt seen in other build logs. Among other things they have much higher rigidity and lower runout than any router, including the Kress. For cutting steel you need quite high torque at a low rpm, which is something most high speed spindles and routers lack, but to an extent you can get away with it by using small carbide tooling ideally with the right coating.

The obvious problem is you're in a similar situation to me when I starting building the router - it needed to be cheap as otherwise I couldn't make it. For that reason alone I started off by using a router for about £6 from the local car boot sale. It was fine for cutting wood at a modest speed, but no chance of cutting metal properly. After (approximately) a few months I did break it by being a bit too ambitious with the tool length, but by then I'd done enough with it to persuade the 'Bank of Mum' that it was worth getting a better spindle, so after a similar discussion on the forum I got the 2.2kW water cooled spindle. The difference in performance is huge.

So basically I think you should try a very cheap router to start with as that'll be fine for making signs and other bits of pieces from wood, then you can re-invest the profits in getting a proper spindle and start cutting metals. Unlike the rest of the machine you don't loose much money by going for the very cheap option to start with and still have a useable machine.