For a skimmer for a reef tank you want it to be rated for about three times the actual tank volume. Most people do not consider the Coralife to be worth the money. Look into Reef Octopus, they are a great option and are reasonably priced. Marinedepot.com is good to check out for comparing equipment.
The maximum return pump you can get is whatever your overflows can handle. If your overflows can handle 800gph make sure your pump does not exceed this AT THAT HEAD HEIGHT. Remember that flow is reduced with height. Most pumps will have a chart stating the flow rate being pumped up 1', 2', 3', etc.
Most 'modern' sumps will have at least three sections. One is the first section that water drains in to and is made to cut down on the bubbles. Ideally this is where your skimmer is or returns water to so that any bubbles that escape the skimmer can be handled before they hit the return pump to the display tank. This section may contain live rock in some sumps. The second section is usually a refugium. This is the macroalgae that is grown and removed to remove nutrients from the system. This section will frequently have sand and rock rubble. The last section is usually just for the return pump and any other pumps you may need (I also have one for my skimmer and another for my media reactors). Depending on the skimmer some need a consistent water level so they may not be in the first section.
For lighting I recommend getting high output (HO
) T5, or HOT5/T5HO. These are most up to date but still cheap lights. In your case I would suggest starting with a slim two bulb fixture with individual curved reflector. As you advance in the hobby and get more demanding corals you can add more two bulb fixtures. This spreads out the cost, still leaves you with adequate lighting, and does not provide too much light for your needs. I am assuming this is a 6' 125 in which case you may need to get two 3' fixtures.