you will need at least 766 permissions,
Read 4 - Allowed to read files
Write 2 - Allowed to write/modify files
eXecute1 - Read/write/delete/modify/directory
4+2+1 = 7 etc
Ask your local business link if there is anything like a business incubator locally, or they might know someone who wants to sublet an office (which is what we did for a while - IIRC it was £26pw inclusive)
This document on PCI Audit Procedures is worth a read. It would be worth talking to your clients Bank to see what kind of security requirements they have.
It would depend on what information you wanted to store on the card as you will need to be PCI Compliant which means you cannot store the 3-digit card number from the rear of the card.
A system that we have integrated with an ecommerce website is SalesStream (http://www.jsanet.co.uk/) it is also scalable to include additional shops if the business grows