New 100 Gallon Set Up, Help & Advice Please!

What is your overall goal? That will help to determine what is needed.
A sump is not needed but it gives a place to hide equipment and adds water volume. I am not using mine at the moment due to an odd problem but it was simply an unused tank with pressure fit acrylic baffles.

I would look into getting more/different powerhead options. The maxijet works, but it is going to be woefully inadequate for your tank. I'd look into the seio powerhead line, as well as Hydor's Koralia line. The latter would be a bit easier to work with. That should give you the water motion you'd need quite easily.

If possible, I'd also look into getting a sump, which just involves another container of water in the bottom and a relatively small return pump to push it back into the tank. Do you have any way of having the tank drilled? This route will make things much easier on you in the long run, for sure, as well as opening up lots of other filtration options (particularly, some better protein skimmer options).

A lighting upgrade will also be necessary if you want corals. In short, there are many changes that I would recommend that would make things easier for you before adding corals.

Your filter will be fine and some will tell you eventually it will not be needed. I do keep one running myself though.
Nothing wrong with the Maxi jet power head but you will likely want more. I like Koralia power heads and you will see most agree.
I have no experience with that skimmer but most say it is a bit difficult to use as it needs frequent adjustments. I use a Remora.
Probably going to need more light for any type of corals. I used 216 watts when I had my 55 gallon set up and use 150 watts over my 29 gallon. I have not yet started with corals though so someone else's advise will probably be worth more than mine.
As for live rock....shoot for 1-2 lbs per gallon.

My post got held up. Glad to see Amp had about the same thing to say. Always good advise from Amp!!

If the rock has been kept out of water then it won't be "live" any longer but you can still use it. If you purchase live rock this will then seed the "dead" rock.