Medical schools only require 3 A levels. You need 3 or 4 good (A grade) AS levels to make your (AAA or higher) A2 predictions seem reasonable. If you take 5 AS levels, you are massively increasing your workload and stress levels, making it much less likely that you will achieve AAAAB (and really, you will want to get AAAAB at AS Level if you take 5 subjects). Considering you are recommended to do 4hrs per subject per week of studying, are you really prepared to do 20+ hours a week of work outside of classes?
Medical schools often won't even consider any "excess" grades you achieve, as most 6th forms and colleges don't allow you to take 5 AS levels.
What subjects would you take? Biology, Chemistry, and Maths (or Physics) are the subjects that you need to take to A2 level if you want to keep all the medical schools open to you. Further Maths is NOT considered an extra A level by medical schools.