When it comes to powerheads I like running at least 2 on opposite ends of the tank. On my 55g I ran 3 all rated over 1000gph on a wave maker for random flow. When I upgraded to my 90g it wasn't enough and now run 2 vortech mp40. I kept mine in the top 3rd of the tank pointed so as to maximize flow patterns. One can't reliably tell another how to place and aim them, it's all personal and varies for your tank inhabitants. For example sps requires more and random flow where as softies require less. Rock work changes flow also. It's something you will have to tinker with to keep your personal tank inhabitants happy. I will however recommend that you avoid wasting money on puny little powerheads and buy them based on what you plan to do. If it's sps you plan to keep once your tank matures then keep that in mind buying powerheads, don't go buy the highest flow ones either but focus on getting the desired flow based on what you plan to do. For FOWLR you want minimum 10x tank volume in flow so on a 20 gallon tank you want at least 200gph total combined flow. For softies and LPS you want at least 20x flow so on the same tank you want minimum 400 GPH total. For sps you want at least 30x flow, so you would want at least 600 GPH total. With that in mind you can properly size your powerheads. If you start with 2 powerheads rated about 200-250 GPH you would be able to keep most things alive and happy and be able to get a good flow pattern. Then when you jump to keeping sps just add a 3rd similarly rated pump and run it on a basic timer to get random flow.
When it comes to filtration I would skip the HOB filter especially if you have a skimmer. Filters tend to be a hot debate in reef keeping due to their trapping detritus. With this they require frequent cleaning to remove waste and other build up before it starts to decay which creates ammonia, nitrites and nitrates. A skimmer will accomplish this same function however it completely removes the detritus and waste from the water column into the collection cup which you then simply dump and rinse. If you need added surface agitation just aim a powerhead up a little, if it's for adding oxygen to the water column then it's not needed since a skimmer agitates the water with air oxygenating far more than a filter would.
