Welcome to the Forum.
I made my own 3x8 C&C cage with coroplast from a home improvement store and Amazon Basic grids for my three girls. It works great at keeping the hay in and is easy to clean. Have you looked at Kavee cages? I know they are available in Europe, but so are other brands of C&C cages.
It's great that you want to get a C&C cage and adopt more guinea pigs.
The size of the cage depends on whether you have males or females though, along with how many guinea pigs can live in the cage together.
Males should be kept in pairs, so you would have two pairs of males that live separately, they should not be kept in a trio and definitely not in fours.
While if you have females they could live in a herd of three or four, of course, it's not always a 100% confirmation that it would work.
2x5 is the recommended size for two males. That would be two separate 2x5 cages if you want three males.
A 2x6 is the recommended size for 4 females.
The guinea that
@Bill & Ted linked has information about cage sizing.
Here is one about adding more guinea pigs.
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?