quarter of an inch good, also the sump is going to be 24 long 15 high 11 from front to back is this sufficient for a 55 gallon tank and the last question is about the baffles what size and placement this will be well appreciated
baffles should be spaced at least an inch apart. There are differences in Acrylic, but most of these differences have to do with clarity wich is not as much of a concern in a sump. You want to avoid the Plexiglass brand sold in HD, as it gets brittle as it ages.
Are you building the whole thing from Acrylic or are you using an existing tank and putting baffles into it? If you are using an existing tank, Glass or Acrylic?
Get CAST acrylic. don't get extruded (it will soak up more water and expand more, stressing the seams.) In sheet goods it's only a bit more money for the cast.
Baffles are up to you. I have three in my sump. Over-Under-over is how you want the water to flow. Depending on your flow through the sump you might only need one. Figure out how high you want each water chamber and build the baffle accordingly.