I have had students use exclusively Khan Academy for content review and they were able to score well! However, unless you're on a time crunch, I would recommend finding some extra question sources in addition to AAMC. I would suggest this for two reasons:
1) You want to hone and practice your strategy on other material and then once you have that all figured out, you can use the AAMC material to fine tune things. You want to learn a bulk of the material before getting to the AAMC material. You don't want to use the AAMC material to figure out your strategy. You want to have your strategy set and then make minor tweaks when using the AAMC material. That way you get the best use out of it!
2) Although the AAMC material does have a decent amount of questions, I hesitate to say it's solely enough to fully prepare for the MCAT. It definitely is the most representative and best material, but I always suggest to my students to use other material to supplement the AAMC material to ensure that they see all the different angles a question or concept can be asked from and the AAMC material just isn't extensive enough to cover every topic. The more ways you see a concept or question tested, the less likely you are to be surprised on your exam. For that reason, a third-party source of questions can help fill in those gaps.
I have a link in my signature for the Next Step free practice bundle, so you can give that a look before having to commit to anything! I know it's hard to come by money to pay for extra materials, but I would say that the investment is worth it since doing well on the MCAT will be a major gatekeeper for medical school and if you do well, it will open a lot of doors for you and that would be money well spent. Of course, do your research on what materials fit your study style and timeline best, but if you have time and money isn't extremely tight, I would look into third-party sources as well! If you have any questions at all, please do not hesitate to ask!