Have a new IM 20 (Peninsula) running and here's my opinion:
UV Sterilizer - Might be worth having on hand for certain circumstances, but I don't think it's a critical purchase initially. Maybe if you have dino or algae problems that you're having trouble dealing with.
Skimmer - Eshopps skimmer seems to be run in a lot of successful small IM AIO tanks. I plan on getting one if I have nutrient problems. Currently trying Purigen, matrix media, and staying under my tank's "max" fish capacity to avoid needing one. I don't think you can go wrong with it though.
Mini media reactor - What are you hoping to run in it?
Chemipure - Pretty standard filtration component in a lot of tanks.
Bioballs - I think Seachem Matrix will give you more surface area. Extra biological filtration can't hurt if you have the room. Plus, if you need to set up a QT tank in a pinch, you can take the matrix (or whatever biological media you have) and drop it in the QT tank to have some bacteria established.