I have a ford mondeo 2004, with ford cd player 6000. I want to put an aftermarket radio player, sub 100 euro category. Any help? I must buy a double din and a fitting kit??

I doubt you'll get a double din unit for less than €100. You need either a single or double din fascia (if you are prepared to dive into the used market), wiring adapter, and stereo of your choice.

So in order to work, should I buy this kit (https://www.dynamicsounds.co.uk/ford-mondeo-car-stereo-fitting-kit-wiring-harness-aerial-removal-keys-473731984.html) and a double din unit (like this one: 7010B Multimedia Player) ? 

You also need the fascia - like this. I'm pretty sure this is the correct one for a mk3 mondeo.

That double din unit is cheap crap from wish. Honestly don't buy it, it's a complete waste of money and either won't last, or be so buggy that it will annoy the heck out of you. The screen will be hard to navigate as it will be so slow to respond, the sound quality will be abysmal (you can't get a decent amp for £30, especially when you're getting a touch screen for that price).

Like I said, in order to get a decent double din touchscreen unit you're looking at around £200. For a new unit, any cheaper will just be utter rubbish.