Baffling your sump will allow you to trap micro bubbles before they have a chance to get to your return pump where they would then pour into your display tank which would be aesthetically unpleasing. Baffling will also give you chambers of water that stay at constant levels, with only the final chamber housing your return pump fluctuating in level as water evaporates over time. Housing your skimmer in a chamber with a constant water level would be a benefit, as otherwise you would have to frequently fine tune your skimmer to maintain the proper water level within it.
As for your return pump GPH, this is heavily dependent on what you're using your sump for. Assuming you already have suitable water movement in the DT via power heads and the like, and you are essentially only using your sump as housing for your skimmer, then the GPH need only be high enough to keep your skimmer running effectively. For example, if your skimmer is running at say 200gph, then running 1000gph by it via your return pump is overkill by about 800gph. In contrast, if your skimmer is running at 200gph yet your return is only moving at 100gph, then you've effectively cut the performance of your skimmer in half.
Remember that for whatever pump you buy, it will run a lower and lower GPH the higher you have to pipe the water to get it back into your DT due to pressure. This is known as head pressure, with the 'head' being the height at which you're pumping the water up to. So a pump that is rated 350gph may only be capable of 200gph at the top of your DT. Most pumps will provide the expected GPH at 4' head, the typical height of the average aquarium.
Your overflow box will need to be capable of handling a higher flow rate than what you will actually be providing, as a safety precaution. Matching a 800gph overflow with 800gph worth of actual flow is a recipe for disaster.