Obviously if you're equipped to do it with a rotary and it'll produce a respectable job then that's the best course of action. But if the soil level is demanding of HWE rather than a bonnet system you've got to do it really!
I wouldn't be afraid of using HWE but it needs to be done properly. Yes airflow is an issue but it isn't insurmountable. Free-rinsing non-detergent products are best because they require less rinsing to extract the soil.
You're also at a serious disadvantage if you don't use a glide on your wand, as this helps you to maintain air flow. Since using a glide I really don't know how i managed without one on low profile!
Allow to partially dry after extracting, lightly re-spray with non-detergent product and bonnet over. Then get loads of air over it to dry it as quickly as you can. The bonnet coupled with fast drying should avoid wicking.
You use a portable setup if i remember right? If you do HWE this you'll have to be going at a fair old whack to do it in 2 sessions of 4 hours each! You've got to clean 66 metres an hour, and that doesn't take into account setting up twice and packing away twice! I'd definitely be allowing more time than that for extraction with a portable, but you might have enough time with a bonnet system, depending on how quickly you can work.
Offices can be a bit fiddly and time-consuming, going round desks etc... incidentally have you measured the comlpete area of the room or just calculated the actual amount of carpet you'd be cleaning? What a lot of people do is to quote for the complete floor space and not move desks and cabinets, so you're not actually cleaning all of those metres you're quoting for.
Paul I'd consider your pricing very carefully! Maybe subbing in another cleaner might be an option if you ned to get it done quick. Of course if they don't mind how long it takes you then bit-by-bit in the evenings shouldn't be a problem. But remember you'll be having to set up and pack away each time, which when you consider you'll be carting things up and down stairs too can take up a fair amount of time and gets a bit knackering!