Casual AutoCAD seems to mean that performance is not an absolute must.
Light/mild gaming generally means you will not be playing graphically demanding games especially considering you will be relying on the Intel HD 4600 graphics core. Most games do not use more than 2 cores. Even some "AAA" games still uses only a single core like the entire Assassin's Creed series.
The Core i5-4200m seems to be best suited for your needs.
The Core i3-4000m lacks Turbo Boost so the CPU does not overclock itself to provide extra performance. However, it is good enough for your needs if price is a major concern.
The Core i7-4702mq is a pretty powerful CPU, but based on your needs it is not necessary especially since it will consume more power and reduce battery life.