OK, thanks for that.
The intention is that the fan will be blowing air
onto the carpet, with cool air coming in through the (ideally filtered) front air vent. I'll still have the normal dust problems, but they should be tolerable.
There's a photo of the res I'm thinking of using
here. The idea is to mount it vertically into the top of the case, with the lid being removeable for filling. I was more thinking of if this would be a problem to drain the system to change coolant - the thought of holding a PC upside down over a sink doesn't appeal!