<opinion> First off, good choice on Empress. They build some high quality effects--although I dunno about running an all empress board :D--Seems a little thematic to me, but totally cool.</opinion>.
Ah the power supply question. Everyone needs them, nobody wants to pay for them--so they resort to wall warts and daisy chains. Let me be the first to say that I absolutely hate wall warts. I'm a neat freak, so my board has to be as aesthetically pleasing as possible. I guess I should spend less time obsessing and more time practicing--maybe I'd get more gigs that way :D. Anyway there are two things to pay attention to here when selecting a power supply. First off, you need to know the current draw in milliamperes (mA) of the pedals you plan to power. Voltage is simply potential, whereas current is what runs a pedal. Most of the power supplies on the market will give anywhere from 100 mA to 300+ mA of current on a per port basis. Next, you need to make absolutely sure that your power supply is rated to give at least the minimum amount of current that each of your pedals require per port. Some non-isolated power supplies rate themselves as the maximum current draw across all ports, so you have pay attention to that. For example, if a PSU states that it supports 500 mA of power across 5 ports, then that means you have a maximum of a 500mA current draw on the supply and each port can support a variable amount of amperage. If your combined total current draw on all attached pedals is greater than 500 mA, you'll notice some sag, but if it's less you'll be fine. A disadvantage of supplies like this is that you don't get completely isolated power lines to each pedal, so there's a greater possibility of ground loops because it's basically a daisy chain inside a box. You won't damage a pedal with current starvation, but you will get some funky, saggy sounds. Note that this is desirable in some situations like fuzz pedals--hence the sag ports on the PP2+--but if you have sag you definitely want it to be intentional.
So for 12 volts DC you have a couple of options. I use a T-Rex Juicy Lucy to power all my 12V AC/DC pedals (I have five of them too), but unfortunately it looks like it has been discontinued. You could go with a T-Rex Chameleon instead--which would give you many more options if you ever ditched a 12 volt pedal or needed 16V or even 18V options. So far I haven't really found anyone who builds PSU's like the guys in Denmark. I use the ubiquitous Pedal Power Plus for all my 9 volt stuff (great unit), but no one on the market builds a decent isolated 12V AC/DC supply like T-Rex. The dimensions on both of those units are also very nice for tucking underneath your board with some Dual-Lock. In the event that you have a high draw pedal, you can use a current doubling cable. This will in effect double the mA that you can deliver to a pedal at the expense of taking up two ports on the supply.
To fix the polarity issue, a simple inverting cable like mrbuxley linked you to will totally fit the bill. T-Rex also sells them.
Finally, let me be the first to say that PSU's are expensive. Like, really expensive. But, as with most things you get what you pay for. It's similar to purchasing an amplifier for a home stereo system. My friend and I used to joke: "The heavier the amplifier, the better the amplifier." Although we were just kidding around, there's a bit of merit to it. A powerful and well built amplifier will be heavy, and expensive, because of the nature of the components involved in building them. The same goes for power supplies for pedals, or really anything. Although the application of powering guitar pedals may have some special problems (such as eliminating ground loops etc.) in the end, it's just a bunch of transformers. Those transformers must be made well, and up to international standards in order to work. Components designed to manipulate electricity to our pedal's whims are not cheap to build, but they are completely worth it in my experience.