Hi Sean Starting from scratch I would go for the latest ATL Automated Test Lane system may cost a bit more but does away with the assistant and reduces your labour costs. I agree with Peter Bosch equipment is usually dearer to purchase but very much lower to maintain as I have gathered over the years.
Inquires to VOSA will keep you right and ask about class 5 because I think you can test class 4 on the same equipment. Peter mentioned class 7 this class requires longer ramps or pit and includes coaches i.e. private owned not under PSV category.
Look at their web pages
http://www.vosa.gov.uk/vosa/mottestersgarageowners/mottestersgarageowners.htm
The computerized system is very simple the time per test averages .75 hrs but as you know the 3 year old 12,000 mile Polo is going to be quicker done than 200,000 12 year old Cavalier.
HTH

Iain