Just to be clear, you're buying a 220V 50hz tape deck / mixer and want to use it in a 110V 60 country? Where are you located? All you need to get is a step up / down voltage converter, one which gives you a good 3x the total watts of your tape deck / mixer. For instance, if the tape deck is 100watts and the mixer is another 100watts (I'm guessing 'cause I don't know) then the total watts is 200watts. You need to get a converter which is, ideally, 600watts (or less depending on actual watts of your gear). This gives a good buffer zone and doesn't overwhelm the converter. I think your Fostex is all DC so it doesn't regulate its speed from the AC outlet but I could be wrong, meaning that it really doesn't care about the 50 vs 60 hz.
A couple of hints: check the WATTS on each piece of gear and also if it's SWITCHABLE (some provide voltage switch).
But the question is.. Why are you buying 220 gear in the first place?