How bad is it to only have taken a max of 2 courses/semester in a premed post-bacc
I think you are paying too much attention to how others will interpret your workload and grades, and not enough attention to what you can learn from them.

The money issue is irrelevant because you would be prevented anyway from taking more courses because of the time you need to devote to orgo II and mathbio. You are considering dropping from three courses to two because of the workload, not to save money.

Something needs to change before you would be ready for a demanding graduate program such as an MD/PhD. The question you need to ask yourself is what are the reasons for the difficulty you are having handling three courses, and are those reasons something you can change in the future.

For example, if the problem were a full time job you could solve it by planning to be a full time student.

If you can identify a fixable problem you should do two things: 


Fix it ASAP, so that you can see and show what you can do with the fix in place.
Discuss the problem and its solution in your application materials.


If you cannot identify a fixable problem you may need to reconsider your career path to pick one that does not require the sorts of courses that are giving you trouble.