Sounds about right, but you can get by with 14ga. plate under your cylinder. Way easier to cut and over that span will hold several hundred pounds. The devil is in the details, especially with tubing. So work out where you will use mitered joints and where you want butt joints, so that any tube openings can do dual duty for something or be capped or covered. Some creative carts have the tube legs open for handy rod storage, but that doesn't really apply for a MIG cart. Be sure to provide a place to store your cables and having consumable storage is a big plus. Expanded metal shelves are nice in that they promote great airflow for cooling and don't catch crud. Of course then you can't put small items like contact tips on them, but depending on your design you might have a small drawer for those things or keep them in a nice divided plastic box.
Everyone has a different idea of what is their perfect cart. Someone on this forum made one of the most unique ones I've seen.