Just because some of you are asking me where to find a Neptuno FPGA, you should understand that this group is not a FPGA seller or provider.
Kyp uses the Neptuno for his own convenience, because it has composite video output and is fast when synthesizing the "core" of the computer(similar to compiling a program). It could be that the definitive implementation would be bigger than the Neptuno possibilities, so better keep calm and wait for further development.
If you still want to enter just now in this new world, the target FPGAs for the Enterprise will be these with enough capacity to be valid for some years. Yes, they also get obsolete, because some cores continue growing, or because other computers more complex are tried. I am writing here "FPGAs" in plural, because another speciality of this group is to port the cores to all available FPGAs with enough space.
The EP is a computer that, except for the Z80 or the WD177x, has no custom chips in common with other computers, so its development will take some time, because it shares less implementation. This means that the size of its implementation could be bigger than other contemporaneous computers, because these tested common modules can't be used as bricks to build the Enterprise.
The FPGA contenders can be Neptuno, MiST, MiSTer, MistiSIDI and other new ones that can appear on next years. But the actual chip crisis has also raised the price of these programmable chips, so it is not recommendable to buy them now.
Here in Spain there are two main FPGAs builders, Antonio Villena and ManuFerHi, but I think you have a builder in Hungary, I don´t know his name.
On the other side, I am not an FPGAs specialist, there are other better forums where to find information about the subject.