Hi Paul,
WFP is fine with these windows, just check as you work and take the pressure into consideration when rinsing. We use WFP on a lot of old georgian style windows, just comes with experience and trial and error.
Regarding the pricing. I personally would work on your hourly rate and add 20% to the final bill for the unexpected. These jobs are more valuable for status than for financial. You will be surprised how much more work you will obtain on the back of this client.
Name drop is so important when quoting like for like, it builds reassurance and trust with future customers.
Good luck,
Trev