Your first is your worst. Feeling anyway. I drilled a hole into the back overflow area of my 60 cube for the return. No issues. When I drilled holes for my ghost overflow on my 220 one of them had a huge chip on the inside of the tank. I used painter's tape to make a dam and windshield repair epoxy to fill the chip. Never an issue 2.5 years later.
There are risks, of course, just do your best to no rush it and apply any pressure. Let the weight of the drill do the work while you focus on making sure it's as level as possible and well lubricated.
Regarding AIO's, I started with a 32gal BioCube and loved it. As long as you keep up on water changes with that size tank and run an ATO, there's practically nothing else you need to do. I'm actually thinking of getting another as an upstairs tank. Nothing wrong with preowned as long as it holds water and the silicone looks at least decent. My current 220 was pre owned. Yes, it has a few scratches, so look for that and decide if it's a deal breaker for you. Nobody has ever commented on my scratches(which aren't bad), only on the livestock.
For cycling, I've used pure ammonia and bacteria in a bottle. I usually buy 2-3 kinds of bacteria products in the hope that I'm introducing some diversity. If you're feeling frisky and patient, get some live rock from a store, or even better a vendor that farms it in the ocean, and start it off that way. Fair warning, you will need to monitor for pests like mantis shrimp, crabs, and other pest hitch hikers, but the biodiversity from the ocean is unparalleled. For LR from a store, keep the tank fallow for 72 days after adding the LR.
For general equipment, you'll need the tank, sump(unless AIO), return pump, power head, and light. What type of light and power head(s) will depend on if, and what kind, of corals you want to keep. You can buy a pre made sump or build your own. I've always built my own as I'm too cheap to pay 400 and up for something I can make for well under 100. You'll likely want a protein skimmer as well.
For general advice? Take it slow. Be patient. Buy once, cry once. Don't stay away from used equipment, but be careful. Research, research, research. Don't be afraid to ask any question here, but do try to use the search feature first

. Have fun! It's a wonderful and rewarding hobby.