Rayzor, all the information and suggestions Kathy has provided are spot on.
The technical term for a protein skimmer is actually foam fraction
ator. This piece of equipment removes dissolved organic compounds (DOC's). As Kathy stated, it also oxygenates your water and does much more good than bad. There are certainly reefers that don't run protein skimmers, but these are historically more advanced reefers that have experience with saltwater husbandry and more specifically successfully maintaining coral and inverts. IMO you would benefit greatly by running a skimmer.
Using a sump with your system and using an external overflow such as the Eshopps or CPR overflow boxes requires more precise installation and finesse than running a drilled tank. Certain information you've been given needs to be corrected. I'd hate to see you have a flood based on it. You really should consider using an overflow box with a higher flow rate than 300 gph. It is
IMPOSSIBLE for an overflow box to overflow water faster than your return pump puts it into the tank (providing you have no leaks in the tank). Therefore it is of utmost importance for you to pair your return pump with a larger overflow box than your pump can return. For example, use an overflow box that can handle 600 gph and a return pump that can only return 500-600 gph at the elevation of your tank return with backpressure taken into consideration.
The API Saltwater Master Test Kit is fine for you to complete your cycle. I would NOT use the API Reefmaster Test Kit. The
Salifert individual kits are much more precise and appropriate for maintaining a reef.
I hope this helps. If you have any questions going forward I'd be happy to help.
Russ