Right now I’m looking for mask, fins, snorkel, and boots.
Is it better to rent or buy your air tanks/cylinders?
Also, it needs to be affordable, because I’m not looking to be completely broke, like at least under $300.

In all reality, no scuba based manufacturer can afford to make bad scuba equipment. I’m sure that you have already been made aware that comfort and fit is the main priority when it comes to purchasing scuba equipment. Scuba equipment refers more to the regulator, buoyancy compensator and scuba cylinder. The mask, fins, snorkel and boots, though used in Scuba, are often referred to as snorkeling equipment because they may also be used for the sport of snorkeling.
Be careful though. Masks and fins that are considered suitable for snorkeling may not be suitable for Scuba Diving. You may want to read some of the guides on snorkeling equipment at this link http://www.snorkelingonline.com/snorkeling-guides.html the guides will give you a better understanding of the different types of masks, fins, snorkels and boots that are available so that you can make a more educated decision on what you want as well as what works for you.
As an example, because I have a moustache, I prefer a low volume mask which is easy to clear. Low volume masks are traditionally the two window masks which have 2 separate viewing lenses
( http://www.visibilityunlimited.com/scuba/Dive-Mask ) for easy clearing. Please note that what I like may not be what you like so reading the articles will help.
You may even find a complete set like the one here http://www.visibilityunlimited.com/scuba/Mask-Fins-Snorkel-Sets or here http://www.snorkelingonline.com/snorkeling/Mask-Fins-Snorkeling-Packages.html that may be something that suits you.
As far as buying your cylinders versus renting them, if the bulk of your scuba diving involves packing your car and driving to local sites, then yes purchasing your cylinder could be a good idea. If you do more diving in areas which require you to fly to them (Tahiti comes to mind) or if you are planning occasional diving then renting would probably be a better option for tanks.