With the empeg-only solution you'll need to get some power amplifiers. Your amp can have any number of channels depending on how you want to drive your speakers. You just have to make sure that the amp can deliver enough power to the speakers to provide you with the volume you want.
There are 1, 2, 3, 4 and 5 channel amps. You can use any of them to drive any combination of your speakers. The empeg itself has 2 pairs of outputs with a fade and balance control.
The empeg tuner will not complicate the line-level connections to an amplifier. Hooking up the empeg in this case is a little more complicated than a regular head unit, but not much. The empeg alone is pretty much like hooking up any other head unit. For specifics, see the FAQ as Tony suggested.
The only complication you'll have with an empeg in the UK is that you'll have to pay a little more shipping to get it than someone in the USA (the empeg international store, based in the UK, is sold out of empegs). Check the forsale forum for sources of units. Jeremy of carplayer.com is selling new ones at prices currently below those on eBay.
Bruno