Are you thinking “kitchen” mixer or commercial mixer? If commercial, which are really big, really powerful, and will produce a LOT of batter for those brownies...check out your local restaurant supply store (may or may not be one near you, but you can always go online or phone the nearest one). They can tell you which would be the best for your home business.
If, on the other hand, you’re not increasing your output that much, and only need a kitchen mixer to get the brownies made faster and at double the usual amount....I advocate for KitchenAid. They’ve been the workhorse for kitchens for literally 100 years (the first ones came out in 1919). They’re super reliable, last and last, have a good reputation as a company for helping you out if something breaks. They’re also recommended by Cook’s Illustrated, and they can be modified with extra attachments to do all kinds of things (like a meat grinder or pasta maker).
Their largest size would probably be best, the mixer where the bowl comes up to the beaters rather than the beaters being lowered down in. It’s the most powerful and professional, but also the tallest, so be sure to check the height of your counter and shelving to see if it will fit. Speaking of which, they now make a “mini” mixer if your kitchen is super small. It has all the power of their big ones, but is a third smaller and lighter.
An added bonus: KitchenAids come in pretty colors
One big warning, however, if you do go with the largest model KitchenAid. The big ones come with these lovely glass bows. If you’ve watched any baking shows on the Food Network, you’ll have these seen these type of KitchenAid mixers and their glass bowls. But in almost everyone one of those shows there’s an episode where the bowl’s lip ends up breaking and ruining one baker’s cake or cookies. So if you get the big mixer, be sure to also get a big metal bowl (or two) to go with it. The glass bowl is good for show, but precarious as your go-to bowl.