How many machines are you planning on running?! That transformer is rated for 2500VA, so since 625VA is suitable for 4 motors, it could power 16!

From that site this one would be the best option:
Standard Range Toroidal Transformers: CM0625225: 625VA 230v to 2x25v
If you're planning on adding a 4th axis then the 750VA isn't much more and would allow you to do that. It's better to get the 2x25V transformer rather and put the windings in series rather than 50V and put them in parallel since if the secondaries are in parallel some current will flow between them (and not into the load) due to manufacturing variations, which could be harmful if significant. It's not very likely though. Another option if you do have to use a 50V transformer is to not put them in parallel and keep 2 seperate circuits with 2 drivers on each.

When you include postage and VAT this one is cheaper than airlink:
Toroidal Transformer 625va 0 50v 0 50v