I would rather do 3 phase than ring mains or lighting circuits in houses anyday, a three phase is a simple radial and a cooker point I can do in 15 minutes including strip downs and RCB's and neon's in circuit. Turn the power off all phases and then link in the MCC the three phases and earth, go round the motors and test R1 and R2 for each phase at the motor.
For insulation resistance do you test the cable from the isolator to the motor? If yes, and you should, the issue becomes that you have to disconnect every set of motor connections but if not turn off the isolator and test from the contactor in the MCB.
I once worked with an electrician who could do 100's of drives per day using these 2 tricks, remember insualtion resistance can be on the whole site and not simply each circuit but I have never been sure how you would log it on the current forms. The MCC's had test facilities so 3 phase power off control power on, switch every contactor to on and then he tested insulation resistance from the distribution bars. Linked the earth to the bars and then did R1/R2 for each phase to earth at the motors. Remember insulation resistance is over 2 Mohm is fine. I was never sure but he always found the problems before he powered up and always said he could never miss a circuit which we could doing individuals.
Price is always what you are comfortable with, your location, your competition and can it lead to more work. The worst part on housing after a faulty ring is the question "how much to fix it"! So I always invariably find I have already sorted the problem whilst testing.
