What brand of fixer should I use for my BW development?

It doesn't matter. I have used probably around of dozen different types of fixers during the years and I have never noticed any difference between them. They all seem to work just fine. I try to use known brands such as Kodak or Ilford, but mainly I just compare the price per liter and try to take one of the cheapest ones.

The fixing time is not that critical either, just make sure you fix for enough time. I usually fix the film leader I cut off when loading the film into the tank while I do the development. Then I take twice the time that it took for the leader to go clear. And nowadays I reuse the fixer until the fixer starts to smell/goes murky/has particles floating around.