Its not that straightforward yet, but there is a whole bunch of companies who will do much of the work if you dont want to.

You can get PCBs made up in small quantities at ok costs then you need a bunch of equipment to solder chips on. For small quantities you can do it manually. Then when you are happy you can go for volume, with automated insertion. We did this some years back in a startup.

But you can probably find an existing platform that has what you want if you dont want big volume. Eg this has two ethernets http://www.newit.co.uk/shop/proddetail.php?prod=DreamPlug not sure what else you need.

For power, you could use a pc style power supply. You can get them in lots of sizes and voltages.