Biesse Rover 18 CNC router

Do not overthink this, just order mesa cards from masanet.com or mesaus.com or eusurplus.com since you are in Europe, and start the retrofit.
In the meantime study the machine indetail, make notes of things of interest like : the machine has several spindles but only one VFD to control them, check the relays and what and how they control things like solenoids (air valves), check the limit switches and other sensors etc etc. Get to know what you are dealing with as it will come handy later on.
The best thing about linuxcnc is that it is open source, it can be modified and programmed for almost any machine and it has the best techical support of very competent users on this forum.
I could not get a Rover 346 even at 9000 euro and with mesed up computer, since i have seen Bevins ( a user on this forum ) do a retrofit to the same machine i have been looking to find one as i really like the hardware on them.
Then again, to many projects and not enough time.

Todd, I don't underestimate the time needed to update the machine.
I don't need the machine, it's purely a bobby project.

The only thing is, I'm new in LinuxCNC and it looks complex at first sight.
I'll try to install a Linux machine with the LinuxCNC in the next couple of days to find out how it works.
There is probably less complicate software on the market, but I don't know if they can handle a machine with analog drivers... We'll see

And is the PLC logic programmable within LinuxCNC? Or do I need another controller to handle this jobs?

Linuxcnc does have a built in soft PLC program. (ClassicLadder)
If the machine has a hard PLC separate from it old controller, It may be possible to interface it with Linuxcnc.

Linuxcnc is complex, but that complexity is what makes it possible for it to be used on machines that are very complex or unusual. Where other simpler controls (like Mach3) you would have to give up some of the machines original functionality or safety features.

I've just installed LinuxCNC on a virtual machine on my Macbook. It looks very promising !
The next step is to order some Mesa hardware en do some tests.

The Pin and Step wizards are very handy for a LinuxCNC dummy

Maybe you already know this, but VMs are great for experimenting with the software and simulations, but they can not be used for controlling real hardware with Linuxcnc.

I'm already looking for a PC with a parallel and a PCI port
But to evaluate the interface and do some simulations, it is nice to have it on my macbook