Are you doing the whole lot all in one go or a certain area? Just bedrooms or corridors and communal lounges too? Maybe they don't know what they want done yet, they might be "seeing how much it costs" before they decide.
21 bedrooms, plus corridors plus lounges etc, gotta be a few thousand square feet at least. Big job if you get it all, certainly way more than one day solo with a portable, probably 3.
At £1.50/metre you may find you're ripping yourself off. As said above, this is one instance where I've started charging for time rather than by the metre, because it might take me two hours to clean 30m of carpet spread between four bedrooms all round the building. If you're doing a straight run all the way through an area it may be better to charge per metre though.
You'll maybe find your productivity a lot lower than you'd like, if it's the kind of place where you have to cart all your stuff backwards and forwards all day. I have this problem with a home that I've done a lot of work for recently. I've tried to get the manager to have certain areas cleaned at one time but he keeps just presenting me with a list of room numbers all in different places. This means I have to effectively set up and pack away for each one, dragging everything around with me. Good job most of the rooms can be accessed from outside!
I'd definitely recommend leaving your machine outside near a window and running the hoses and cables in. That will reduce trip hazards, keep the noise down and most importantly stop you from blowing pee-smelling air all round the building.
Oh and you'll have to disinfect all your kit and afterwards, it'll smell like an old man's pants. Not only will it stink your van out but you'll take the odour with you to your next customer - not a good PR move!
I'd have some rubber gloves with you to handle anything like the waste water, rotary brush, bonnets etc. Wash your hands before eating/drinking etc.
Chances are the place is going to smell of urine, when you get it wet it will smell worse. You need something that's going to address that, and something that smells nice to mask the odour during and after cleaning.
I've been getting outstanding results with M-Power and the odour neutraliser from Solution UK, first extracting after a very long dwell and then re-applying the solution and bonneting over afterwards. Thats both visual results and killing the smell