As with any purchase, you generally want to ask yourself the following questions when deciding to rent or buy:
1) How often will you be using the item per year? How many years? How many total uses?
2) Do you have a good storage place to put it- aka, what are your chances of losing it, or it getting moldy? (Stashing it in a damp area or wet basement or something, for example, is generally not advisable. Needs to be in a dry place to stay good)
3) How much more do you value "owning" the equip over renting it?
For ME, personally, I figure I will dive at least 10 dives per year. I want to keep diving for the rest of my life. I love it. And I like owning stuff more than I do renting it. And I do have a place to put my stuff, thankfully.
So for me, I purchased everything. I don’t rent anything, except the tanks, and even that, I have a standard A-80 tank that I use locally. It’s fun to be able to have your own BCD that you are comfortable with, your own clothing, wetsuit, gloves, hood, fins, etc.
But if I had to say what is most important to own, I would say in order of importance, you should buy these instead of rent them:
1) The BCD, (because you need to know where your emergency dump valves, your weights, and your inflator hose and octopus all are, in a jiffy, in case of an emergency). They aren’t THAT much, you can get a good one TUSA for like $250, and it’s just so much more comfortable and safe having your own. Plus better performance.
2) Your wetsuit and mask and snorkel... for obvious reasons. this goes along with #1 as being something I recommend. Wetsuit you want your own, you dont want to use somebody elses dirty wetsuit and you want a mask that fits your face.
3) Your fins.... pretty much everyone has their own "mask snorkel and fins" and usually renting equipment is a one lump cost for everything. So if you are buyign the mask and snorkel you might as well also buy the fins, otherwise you still pay the full rental fee every time.
From this point on, you start getting into more "elective" optional stuff... like hood, dive computer, regulator and hoses, weights, tank banger, compass, dive knife, spearguns, lights, and a whole bunch of new equipment for cave diving, along with lots of other optional items.
Cave diving has its whole other set of required items you basically need to buy, because few places will rent to cave divers for liability reasons and also them maybe not getting their equipment back if the diver croaks. lol