In a lot of areas it's the MoT which drives the rest of the work. Car owners are putting off routine servicing to save money and the annual test is what drives a lot of business, especially as things like ABS and airbag warning lights are now testable items. If you're setting up a new centre then a one man MoT bay is a must-have, it's cheaper to operate and will feed business to the rest of the workshop.
Diagnostic kit is where it gets really complex, most of the guys I know run three or more bits of kit to get the coverage they need, so you need to understand your local car parc, and the kind of vehicles you're going to be working on to make sure your equipment allows you to do the jobs in house and not have to send them out to a specialist as this eats into your margin.
Courtesy cars are an interesting topic, they can be a big cost and a lot of businesses go down the route of leasing new cars (costs circa £100 per month) rather than running old cars. If you want to do lease company work, which is generally fairly easy stuff (routine maintenance on 1-4 year old cars) you'll need courtesy cars available. Of course, you also then need the diagnostic kit to turn the service lights off, or you'll have to outsource it which will eat all the margin from the job.
As other posters have mentioned, there are a lot of low quality parts around, and many companies will quote based on low end bits as standard. This is something you have to deal with, and a really good service adviser will help here, selling the benefits of fitting a Bosch battery against a white-box one year version or Dunlop tyres against their Triangle or Sunny competitors.
Talk to a few equipment suppliers, and some other workshop businesses, and take it from there, it's not cheap to set up but if you do it right then running a workshop can earn you a decent living.