I have tried to hire people from the other affiliated website. Some of them want contracts, some don't, especially the newcomers who haven't even managed to get their permit yet. If it is a one-off thing, you may be tempted, but you definitely don't want long term unofficial agreements, you have so much to lose if you get caught. It is very important that you have all pensions/insurances paid properly, otherwise you are in deep shit. My employee had an accident at their place outside working hours, the first thing they were asked was the accident insurance of the employer. I had to spend a couple of hours on the phone plus filling in forms with my insurance, for something that did not even happen during working hours, it is not easy...

As a rule of thumb, I would add 20-25% on the quoted price plus a lot of headache to set this whole thing up (or extra costs if you want to assign this to a professional).