I'm not all that familiar with that Fostex deck but, if it's a -10db, unbalanced connection scheme like a lot of the TASCAM RTRs, you'd be best to look for a mixer that has the same connectivity signal chain for it's buss system and tape returns.
The next thing to consider is how many tracks do you foresee yourself recording simultaneously and that number will determine how many busses the mixer should have.
If your working by yourself or with another person, a 4 buss mixer should do the trick. If your recording live bands, an 8 buss or higher board will be required.
In many cases, a mixer will have direct channel strip outs and that can be substituted for a lack of buss outputs.
Also try to find a mixer with at least 24 channel strips so that you have a few extra channels to use as dedicated tracking strips and as extra effects return inputs for different reverb units and such.
Avoid looking at PA mixers as they tend to not have many of the critical features you'll need for recording like the first point I made about signal level and connection type.
Cheers!
