you can also go with a hang on back over flow box so you dont need to drill look at the cpr overflow boxes they are probably the best but i would never reccomend one even though lots say they use them with no problems i never got a good nights sleep ever while i ran one id rather do a tank without but if you need to have a sump and "cant" drill then this is the other option.
as for the skimmer spend the money for a good one and it will be one of your best investments, be sure to get one rated for much more than your aquariums volume and they take a bit of tuning to get either a wet or dry skim, once you do some tuning it could take several minutes for the effect to take place.all skimmers are a bit different so just ask away when you get a particular model
and yes water falls down into your sump and gets pumped back up to the display using a water pump.