O.K.:
First: Stay cool! If you can't do it, it's just fine, give him the unit back and find something you are better at. Be aware, that you can easily mess up the unit so it won't work at all. I somewhat get the impression, that you are afraid of the guys you made the deal with. Right now I suppose there is a good chance for you to end up in a big mess, so think about minimizing the loss. If you need the 50 EUR badly then post your Paypal-ID...
If I am wrong:
The unit basically is a Windows CE computer with a touch screen
well, then just copy the missing files as written in the post (see link provided) you seem to have them
copy "testmode" (folder) to the root of an SD-card
copy the full flash you created to the same SD-card
put SD-card in and start the unit
the unit starts in testmode - open file explorer
copy all files from the internal flash to the SD-card to make a back-up
copy all files from the full flash you created to the internal flash