D.B.
You're right to be confused by the endless aisles of vitamins in any store, whether it's a supermarket or a health food store. These are designed to maximize profits for the companies rather than to meet your nutritional needs!
Your doctor's advice about a "healthy diet" was tossed out by the American Medical Association in 2002, so I don't know why he's still suggesting that. Our foods are so sadly deficient in nutrients due to processing but just growing techniques - so our natural fruits and veggies are up to 40% else nutritious than they were 30-50 years ago.
The problem with most multivitamins is that they are in pill form, which means an absorption level of 15-30% (according to the Physicians Desk Reference, which every doctor has). So they have to be in a highly absorbable form (so said the AMA study), and they have to be at optimum levels (not just the minimum requirements). They also have to be synergistic, which means that every vitamin, mineral, herb, trace element and so on needs to have ALL its essential partners in the same formula! Asking you, the consumer to mix and match vitamins is pointless - they don't work in isolation anyway, and most of us don't have the PhD necessary to do the mixing! As a society, we are choking down handfuls of pills which are not in a usable form and which don't have all the important components. And that's if we know where they are made - which we don't, 95% of the time.
Don't go for something that is just for prenatal, or just for hair/nails. That's not how the body works. Every cell needs exactly the same nutrients for healthy functioning - again, that was determined about 40 years ago. And with all the new work in nutritional epigenetics, which helps cells to function properly by repairing/protecting the covering on the genome (the genetic material is affected by the covering), this is more and more supported by clinical studies.
In my work, we recommend exactly the same thing for pregnant/nursing women as for men and for children. If someone has a particular issue in addition (e.g. joint function, muscle issues, diabetes, cholesterol), the supplements can be enhanced but only by using another comprehensive formula designed to work with the basic nutrition. It's still a whole-body and whole-cell approach.
There are a few tried and true (and proven) ways to evaluate a nutritional company. You should be looking for a very small family of supplements which is comprehensive, not some company that is making a fortune selling you everything possible in a zillion different strengths. It should not have warning labels on it (because it shouldn't be possible to overdose on it - which happens when you take individual vitamins separately), ideally it is patented (proving safety and effectiveness), and it should be backed up by clinical studies done independently by outside labs. It should have the FDA Good Manufacturing Practices designation (not FDA approval, which is for drugs and not for food), and it should be made in the US rather than outsourced. It should not have GMOs. You should have a direct connection with the company, access to a consultant, and access as needed to the research/clinical arm. That narrows the field considerably!
Let M. know if you want more help.