What's the maximum voltage one should wire a pv array to supply to a 24v battery bank?

That depends on what your charge controller is. If you use a MPPT controller, a higher array voltage lowers your copper losses in the the wiring. MPPT controllers should have a chart that shows what their efficiency is for inputs vs. outputs and that should help you chose.

Just the basic, a C35. An MX60 is on the wish list along with more panels.

the other aspect of that question will depend on the batteries used and so the manufacturer will usually give the recommended voltages for each stage of charge and at what degree of change to that with temperature. that means use a battery temperature sensor. for example the max voltage to my sunxtender is between 14.2v and 14.4v (i pick) and the float is 13.2v and then the temperature sensor (known also as a bts) will adjust it from there.

So, what is your charge voltage on your batteries on a hot afternoon ? 28V ? your PC's should put out a couple of more volts than that, when hot, otherwise, you could stop charging. If you are planning on expanding, to get a MPPT controller, you should plan for different wire options in the future.