A step drill is no good for this diameter hole/thickness of steel.
A hole saw will take you most of your life to penetrate 6mm steel.
A garage won't have the tooling to do this job.
In fact other than a labour intensive method, ie drilling a series of 6 or 8mm dia holes to form the rough circle prior to dressing the circle with a flap wheel, it isn't a job for DIY tooling.
I think you're left with 2 options:-
Either buy a 'decent' quality drill (6 or 8mm should suffice) and drill a series of holes (as above and labour intensive) to make the circle. When I say decent quality I mean buy one from an engineer supplier...
or...
ask an engineering workshop to do it for you on their mill/borer or radial drill if the bed is big enough. Circa £20
I would be happy to do this for you but for the problem of getting it to me!
Oh and if you do do it yourself just bear in mind that WD40, as good as it is for its intended purpose, it is NOT a cutting fluid nor is it a coolant!
Also, if you choose to drill a series of holes you would do well to use a multi-speed drill so that you can drill it 'slow' enough. A single speed drill only knows one speed...full chat, which will result in burnt out drill bits before you even get through your first hole!