On the skimmer go as big as you can afford, $400 range for a good one,I run a 40 gallon sump on my 150 but adding a 40 refugium as well pretty quick so I would advise going with a large refugium instead of a sump for a reef tank.IMO
I run a mag 18 for return pump but you will find that once adding lr
you will have dead spots of little or no water movement with only two returns, I added 2 Tunze 6100's and cured the problem and then some but you could go with a few maxi jet 1200's as well. The main thing is buy what you need the first time around to avoid the redoing it later and spending more money thats not really necessary. As far as buying the stuff local or the net I would buy the major stuff online since it's a LOT cheaper in most cases but you have to remember that if the store makes no money they will not be their for you later on, so spend some with them as well to help keep their doors open. I would advise reading several books before going any farther as it could save a TON of money and mainly just do it right the first time. You wil find that you have only scratched the surface of getting started and spending money so good luck and you will soon see that a 150 reef tank is addictive and expensive.