It depends a lot on what are you going to be repairing.
Certain things can need a good deal of other test equipment to repair and verify it works. For example anything that is a measurement device you need some sort of calibration standard to verify it works. For power electronics like power supplies, VFD drives, Servo controllers etc you might need test equipment related to that as well as some test loads. If you are doing automotive electronics you might need a large collection of harnesses to connect to things and some tools to work with CAN and similar buses.
But most of all you need experience with the thing you are trying to fix. It is one thing when you buy a broken device from ebay listed as-is, then try to fix it. Repairs don't always go to plan and you end up causing more damage than good. For your own stuff that is no problem, but when providing it as a service you really don't want to make things worse on a device that does not belong to you. You can get some of this experience in a safer environment by starting off your repair career adventure by buying broken stuff, repairing it and then selling it as working. This makes it okay to mess up and gets you a feel for how much work it is to repair things.