Some great feedback - thanks everyone.
@Cheshireman - I'm not ignoring anyone's advise. I really want to win this commercial contract and am aware I need to pitch the quote 'right'. When I cleaned it as a 'one-off' a while ago it took me nearly 8 hours (quality HWE) and they were very pleased - but they didn't see the bill, I was contracted to clean by their Cleaning Co, who couldn't do HWE.
Buying a rotary will speed up the job, so I can quote a lower price and still make a decent profit. The choice is, should I buy a rotary system (e.g. expensive DryFusion/Thermadry), or buy a simple second hand rotary, just for agitation.