If you're intellectually sound enough to handle the rigors of medschool, you can handle undergraduate courses and work simultaneously. I worked full-time at Starbucks as an undergrad, but I still found first year of medschool when I did not have an outside job at all to be much more difficult. The sheer volume of information and the little time we had to master it was overwhelming. Granted, as an undergrad you want to get As, and most medschools are pass/fail, but you get my drift.
I did know one medstudent who worked full-time as a nuclear engineer while in medschool. He worked nights, he slept four hours, from 8p to 12 midnight, and worked and studied the rest of the time. He had this theory that the body can handle sleep deprivation as long as the sleep it does get is gotten in four hour increments.
I still think he's a maniac. But his story was that his wife, who was a stay-at-home mom of two young children, would accept his going to medschool only if she did not have to suffer a decrease in standard of living while he did it. So he pulled in more than 100K while going to medschool. And he's graduating on time. And she's got her great living standards