Well, usually you should go the other way arround: plan for a calling capacity, then compute the number of seats from that.
20-seater sounds ok, but you need to be sure you can provide with them with the necessary work.(20 seaters FTE can burn about 2000 outbound numbers/day or about 1000 incoming calls/day, but this can vary wildly)
For a cheap setup, technically you are looking at: 2 slightly powerfull computers to act as servers (2000 – 2500 $) and about 20 desktops (see if you can buy them second hand, aim to get computers with PCI soundcards and do not use onboard sound). Depending on what phone lines you want installed, you might need to throw 1000 – 1500$ for an ISDN card, or some sort of PRA converter. For 20 operators, a single E1/T1 line can give you enough capacity( usually they carry 30 voice channels ).
In today’s world that’s pretty much all you need to get started. You can install an open source PBX almost for free and use (also free) software phones. All you need to buy are good quality headsets and some booths for your operators.
I have no ideea about the paper work in your location, but I imagine it’s just like setting up any other kind of business.