I have the WDTV hub and the interface is simple enough for my 6 year old to use it. It is just basic, but you only use for a few seconds it to select what you want to watch, so it doesn't really bother me. If you want fully customizable, then you'd have to build a htpc and install xbmc. The WDTV will go out and find box art for whatever files you put on there (as long as the filenames aren't too crazy). I find navigating the menus to be slow and a little laggy (probably from a slow processor), but it plays 1080p video in any format just fine.
Your setup should be fine, but don't expect much streaming performance out of wireless or powerline. Some people on here can stream wirelessly and powerline, and others have had problems. If you search on here you will find out what I mean. If you want 1080p video for streaming, once you get up to a certain bitrate (that threshold will vary depending on your signal strength, etc.) then it will get laggy. With an average wireless signal, up to 720p will work. Powerline has it's own set of problems and again, it works for some people and others it doesnt. For trouble free streaming, wired ethernet is the only way to go.
As far as accessing the network, the WDTV will be able to access any shared files on the network, and the hub hard drive is shared on the network so you can push files to it from another pc.
What you are doing right though is getting the hub so you can transfer your files to the box before you watch it. If you are staying wireless or powerline, get the hub, or at least get a streamer with usb so you can use a flash drive.
For your wireless router, get one that is compatible with DD-WRT.