bowing of the sump is easily solved by the use of a 2x4 frame around the top of the outside of the sump. If need be, you can even put stands under the brace to keep them at the top lip of the sump.
Keep in mind that baffles do not need to be such that they are anything more than runs of acrylic to increase the length of the path that the water runs (so all the sump stays the same level with ATO). Also, baffles may be made from buckets within buckets if space is a premium. Over-and-under baffles force air bubbles (microbubbles from skimmers or overflow lines) nearer the surface to allow the microbubbles to effervese out of the water column, the same is done with a longer water path. Although using over-and-under baffle plates do this withouth length, and allow for steady pool levels for skimmers in-sump, placing skimmers in your sump may void any advantage gained by shortening the lengths of the water paths, so it depends on what you want (in terms of in-sumpor under-tank equipment) and how much linear room you have.
Rubbermaids are cheap and esy to drill, however, a 55 gal tank is just as cheap if you are patient and don't care how they look, and are also easy to drill and work with. I don't use baffles In acrylic tanks, either, for a number of reasons (mainly because they make sump maintenance a byach...), but it becomes aw matter of personal preferences.
I used rubbermaids or similar plactic containers for sumps for many years without problems. The one pictured below required a wood frame to prevent bowing.
I can't say anything because I have 55 gal tanks for sumps now and run all the lines for equipment into another room for the skimmmers, reactors, Ca reactors, etc...
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