9 fish... well I agree to disagree
Honestly it depends upon the type of fish. Think about how aggressive they'll be, how large they get (NOT how large they are at the store) , are they vast swimming fish. i.e. Yellow tang, blue tang, purple tang, etc... would be very bad.
You can get a hang on the back protein skimmer, I've never liked them myself because if they ever overflow or leak they can make a mess, plus they restrict how close to the wall you can actually put your tank. But if you want a skimmer then it really is your only method. There's one member here who does not run a skimmer at all, and has a very full tank, but we like to say he made a deal with the devil and that is not the norm. If you do go without a skimmer you'll have to do more frequent water changes most likely, although absolute water purity is not that necessary with a fish only system (still the fish appreciate it!).
As for the filtration in general, any hang off the back/canister filter should be fine, just note you will want to change the filter media very often or they can actually do the opposite of what they're supposed to, turning off the filter when you feed is also a good call too. I would probably go with hermit crabs or other animals as the majority of my filtration as they will take care of uneaten food and what not.
As to powerhead, something simple and cheap like the Jebao RW series should be fine, wide flow, they're adjustable as far as speed, and you can set up various pulsing modes without getting waves but getting a little random turbulence.
And you can have a sump without drilling your tank, they make hang off the back overflows, which could be an option but you do need to keep a little more close eye on them (i.e. don't get lazy and ignore them). If a sump is an option you would like to have though I would hold off on the HOB skimmer since it'll be largely irrelevant if you have a sump (and defeats some of the purpose of a sump).
That said, good luck, this hobby can get as expensive as you want it to get, but more likely it will be more expensive than you want it
And you're doing the right thing now going slow and thinking things out before buying a bunch of equipment then going "now what?"