Hi GB,
I work on other peoples bikes and in most cases I work on blocks of 10 hours labour,
So after initial examination/ strip down/ what ever, I prepare a list of parts required and a labour estimate for the owner
I ask for payment for the labour so far and hand the parts list to the owner, Its up to them to spend the time on the internet or phone to order the parts. If they ask for me to do this its charged out at the hourly rate, and money for the parts in advance of the order
Projects can often stall at this stage due to the owners needing to get a dose of reality about the costs involved
Likewise I ask the owners to spend time cleaning shitty parts and do the running around to blasters/painters and so on
In most cases owners are happy to pay this way as there is no major shock to their finances

I have told people to part out their projects and buy a completed bike as it will save them money in the long run

But most have some sentimental (real or imaginary)reason to want to proceed

John