I should add to what Wuv said from personal experience.
If you do not have experience treating diseases, having a "fish rescue" is very expensive, emotionally and financially. I did a simultaneous fish "hospice" - to give dying fish a peaceful death in a home - and a fish "rescue." I used the bodies of the deceased hospice fish to get refunds on both the deceased and the living rescues so that I would not be financially supporting the industry. This also decreased the costs of getting the bettas. However, medications, aquariums, filters, water additives (blackwater, aq salt, dechlor, etc), ended up churning out a MASSIVE bill every month. What's more, dealing with sick fish is massively time consuming. When you have diseases like columnaris or severe finrot, which require a great deal of frequent cleaning, the hours really add up. With my current 9 bettas, it takes about 4 hours to clean all of the tanks (I do 100% water changes as I had huge issues with filters). If I ever decided to go "large scale" with rescue, I don't think I'd have had time for school or a social life! Even now, I have to carefully schedule my cleanings around my 20 cred in college. This is why I have stopped doing the hospice and refuse to have more than 9 bettas at any one time; my plate is full as it is.
So, if you are going to rescue, that is fine; it is your own agenda and you have every right to do it. However, you need to very carefully consider your budget, your time, how many fish you can take at a time, etc. before plunging in to such a project. I wish you good luck in your endeavors, but let me give the voice of experience: keep it small to see what you can handle.
