The intensity of lighting will really come downtown what kind of corals you want to keep. One thing I've discovered in my experience with a nano tank us that the difficulty involved comes entirely from the initial setup. If you take your time and do it correctly there is little additional work related to the tank compared to a larger tank.
My 15g is very low maintenance. I don't have an ATO, so the majority if my time spent on the tank is manually adding water every few days and doing a bimonthly water change.
As for your specific questions: what's your end goal coral wise? I started wanting just a FOWLR, moved to sorties, then LPS and now I have SPS. I've probably lost a few hundred dollars on losses from replacing equipment, but it's all part of the process.
If you want to go with corals, look into a decent T5 system, MH or LED. watts per gallon is archaic and not really used anymore. Set your budget around $200-300 for lighting. Maybe try a DIY LED if you're daring, you'll probably be pretty happy with it! And it would allow you to keep SPS, if that's what you're into.
There's other things you can do too. If I were to start over with a fresh setup, I'd go with "dead" rock vs live rock. Having dealt with aiptasia and gha, peace of mind is a huuuuuge benefit. And it's much cheaper.
Welcome to the hobby
