Sounds like a nice little job!!
Ladders are dodgy Paul, all it takes is one little loss of balance but can be used OK if no other option....
If the floor is fairly clear by the curtains, seriously consider the tower scaffold approach if feasible! It's not just getting up there, it's having all your kit to hand. Presuming your vacuum cleaner hasn't got a 15ft hose you'll need that up there with you, don't try balancing while holding it. Then you need your sprayer and towels to hand, plus maybe a couple of different tools for the vac.
I use a Hunter turbo vac for curtains but that's doing a few pairs of domestic ones - that machine is a bit heavy to lift up and down all day.
I'd definitely be going for the spray/wipe approach, almost zero risk of shrinkage IMO. Normally the back sides (linings) are just vacced, gets rid of said insect skeletons.
Before anything else, give each curtain a damn good shake which will dislodge a lot of loose crud etc. As well as the curtains, I also allow for vacuuming the window reveal or area immediately around the curtains - no point spending all that time cleaning them if they're still surrounded by dust & cobwebs. Explain this in your quote.
I use M-Power @ 1:100 for curtains, does a cracking job and fits the bill perfectly.
You've got about 90m² of curtains there, which believe it or not isn't actually a monstrous amount. I recently did a regular curtain job in a private house that is over 100m. It took me and the mrs about 6hrs, one with the vac the other spray/wiping. Some of those curtains were about 12ft and a real hassle off a ladder but not really a job where you could use a scaffold cos it was a lounge & dining room, so it's a bit of a tricky one really. I used a cylinder vac for that job which I bungee strapped to the ladder

You'll need a hot bath when you get home, and a good rub down from the wife, to ease the aching neck & arms
