Well Seachem Prime seems to be a favorite water conditioner here. Though NovAqua Plus has, hands down, the best stress coat. I don't use it for most water changes because you have to use so much, but if you have a fish with torn fins it is great to use then in addition to tannins. Unfortunately most of the products I like to use are not sold at Petsmart. You won't find Indian Almond leaves or Vita-chem. The first is great for sick fish, or just all the time. The second helps improve fish colors. Petsmart also doesn't sell New Life Spectrum brand foods like Petco. Bummer. :(
Of course the best preventative medicine is clean, warm water. I honestly can't weigh in on what medicines are safe and effective with bettas. Hopefully someone that can will come by. I do believe (and someone please correct me on this if I am wrong!) that bettafix and melafix need to be used with extreme caution or not at all as they contain tea tree oil with suppresses the labyrinth lung of betta fish and other surface breathers.
Oh, though aquarium salt should be on your list if you don't have it already. Epsom salt as well, though you don't get that at Petsmart, just any drug store or Target/Walmart/Grocery store.
A siphon will make water changes easier if you don't already have one.
And of course I am going to suggest frozen foods. I personally love to feed frozen brine shrimp and bloodworms. They are much healthier than the freeze-dried varieties and more appetizing than pellets for a sick fish. Of course if you don't fortify them with something like Vita-chem you will still have to feed them as treats only, but they make a very healthy and delicious treat. On that note, if they have Seachem Garlic guard, while there is some debate about the beneficial properties of garlic, I know for a fact it makes food taste better for bettas. At the very least, mine always go into a feeding frenzy if I add some garlic!
And one last thing, this is a shameless plant plug, but the most low maintenance, fun plant you can get is typically on sale right next to the betta fish: Marimo moss balls!
They help keep the water a little cleaner, many betta fish like to sleep on them/use them as lounge chairs, and some of them are quirky and like to float around your tank. Plus they don't really need anything special to grow like ferts or CO2. Many people keep them in just little jars (without fish or anything else) and change the water every 2 weeks.
Also, lots of people name them and feel as though they have a personality. They are the one living creature that you never have to worry about betta compatibility with!
I have problems with diatoms/brown algae so in my early days I got a scrubber which I don't use anymore because I have a magnet scrubber so my hands don't get wet lol. Scrubbers are alwasy handy to have.
I guess StressCoat is good, I used it for my fin rot troubles and as my main conditioner now. I used to have Prime but when I ran out it wasn't on sale but it is a great conditioner. It gets smelly though lol (sulfur). One of the smaller bottles treats up to 1000 gallons which is great.
For disease, I only ran into Ich problems and some dropsy which I never defeated. My Ich was treated very well by Kordon's Rid Ich Plus which I think has methylene blue. It worked very effectively so that is good to have I guess.
I have Pimafix too but I really never use it because of the whole bettafix/melafix problems explained by Kytkattin.
If you have algae of any sort get a toothbrush sort of instrument. I use this oversized looking toothbrush (it isn't a toothbursh though lol) for tackling algae.
And lastly, try and acquire some sort of QT tank. For my sorority I have a breeders box and just a small 1 gal for my males.
That's all I can think of lol..
Edit: If you are looking for a low-maintenance live plant I recommend the only live plant I have experience with, which is anacharis! It really sucks up a lot of ammonia and waste which is a good supplement but not something you want to depend on. :)
Make sure you get AQ salt!! Thats one thing that you can use for almost anything, especially fin tears.
unscented epsom salt is also good to have on hand to treat problems your fish might have going on inside.
I've never needed to use actual medications though.