Firstly the Pace ADS200 (starting to sound like a broken record here) is going to be selling in Europe soon for approx 225 euro (a little more if you want a setback stand), this is a 120W station. You might be in for some buyers remorse if you decide too early.
If the Hakko FX-951 transformer is rated for both 50/60Hz no problem. If the transformer is only rated for 60Hz then a "rough rule of thumb" is that it will perform like the transformer is 20% underrated, so run hotter and reach saturation earlier.
This is not so much an issue if the transformer was overrated to begin with or had headroom, but it's something to consider if you are running the device at it's maximum rating. Seeing the below picture posted by GreyWoolfe it only states 60Hz 120V operation.
The other factor is the difference in frequency and how it affects operation. As far as I'm aware the carts are the same in both geographies they just have a different series number and a few different tips. The irons are different and that appears to be cosmetic (one has an LED).
Another "rough rule of thumb" is you will need to have a stepdown of at least 2 to 3 times the rating of the device you are running. Refer to the stepdown transformers website/manual for more info on the load.
Make sure if you are buying a genuine FX-951 as there are plenty of fakes out there now.