Firstly you don't require reams of test equipment as many faults are quite simple to diagnose. The more equipment you have the easier it can be especially if you know how to use it but on the whole I tend to get away with the following...
A Frequency Counter. This needn't be expensive, I commonly use a 7 digit Zetagi which cost me £30 secondhand. Try to avoid the 5 digit ones as you cant truly set radios with these, they are only any good to give you a rough guide to where you are. A SWR/PWR meter. Preferably a twin metered one. Mine cost me £21 brand new and needs to be capable of handling 100w or more. A decent Power Supply A 10-14 amp will normally get you by without too much trouble. You can run a few bits of test equipment off this as well as the repairable radio. A multiband radio TX/RX. This is so you can listen to the radio you are repairing/adapting. This can be anything you want, I used to use an adapted president Grant which covered triple to double plus UK40. That made me able to check most rigs out on the blocks. It depends how advanced you want to go. A Dummy Load This is necessary to check the output power of a radio. An antenna will never be a true 50 ohm load all over so this is required for accuracy. There are loads on the market, some you can easily make. The bigger the better as you may well be testing equipment pushing out well over 4w A Solder Station These can be expensive, it depends on where you go for one. You need one with interchangeable tips and also a range of tips as you will be soldering very fine work to PL259 plugs and so on. A de-soldering pump is also a necessity as it is near impossible to get some components out without damaging the circuit board track. A workplace Trust me this is a must. It's no good trying to run a business from your kitchen table. You need somewhere to keep all your equipment which you can just shut the door on in times of stress and forget about for a while. Also stocks of knackered CB's for their spare parts (gold dust) takes up a reasonable amount of space.
Obviously you need an outdoor antenna, switcher boxes and the like so you can set up your shack how you like and user-friendly. A decent benchtop and shelving is a good idea and plenty of power sockets. A comfortable chair is also a good idea. Another good idea is to make up a power lead with multiple connectors on one end for powering all sorts of radios otherwise your going to have loads of tangled cables. An extension speaker is also a must with possibly an adapted plug in end with crocodile clips as ext. speaker sockets aren't always an option on some CB's. An assortment of precision screwdrivers and plastic trimmers will be needed and are available quite cheaply on eBay. Switch cleaner and silicon spray are also very useful and can save having to replace volume/squelch pots etc. that have gotten dirty over time with use. The list can go on and on..... but in general the above should see you well. Many will say you need an Oscilloscope as a must but in reality, I rarely find a real use for them except for tuning SSB radios but this can be very technical and not really recommended for a beginner. That said when tuning radio o/p power its easier to spot spurious emissions (transmitting on different frequencies simultaneously) otherwise you wont notice this unless in bad cases where it will send the frequency counter haywire.
The CB RIG DOCTORS most useful weapon is common sense..... USE IT WISELY
Another point to note is PEOPLE. Some people can be a real pain, some can be completely arrogant and demand all sorts. Some can be great and you can make lots of new friends... unless you bugger up their rigs of course then you can make many enemies. Be realistic in what you expect to charge for your Repairs. Most CB radios these days are only worth around £15- 50 so your not going to make a fortune. An average repair I charge £5 + parts used. If the radio has been severely damaged and it takes a good while that can go to £10 + parts. Adaptions are different. Fitting mid blocks takes me just over 1 1/2 hours including testing so I charge £17 of which up to £9 of the cost can be the PLL IC itself. Fitting Eproms I charge around £26 - £30 but half that can be the cost of the board. Like I said earlier.... don't expect to get rich... it's not going to happen.
Another thing I do is keep a book where I keep information in so I can cross-reference for timesaving later on. Any job you do note things down so you can go back to it later and either correct it or remember it for next time. I write down the positions of the silver horseshoe type adjusters (RV's) so I know what they do without having to try them out as you tend to forget... you...like I... am only human..