Thanks so much for the replies
I'll give it a try with the motu first, the main thing i'm a bit worried about is the direct monitoring because as far as i know, the only way to turn off direct monitoring is via the control panel in a daw. Maybe using an app like rewire to link the ins & outs of the motu (with direct off) via the daw to the in & out of REW would work? I'll have to look into this further!
I have got an xp laptop with in-built sound, but it may be a bit too crappy to use..there's only mic in (not a mic pre-amp) & speaker out on it. I've no problems with grabbing a soundblaster or whatever if needs be though.
In terms of mic calibration would this be a case of inputting the frequency response data of the mic into a text file or somthing that REW can read? I would like to be able to take acurate readings up to 20k if possible so it may well be worth getting the spl meter you mentioned for this, plus it sounds like it'd be better for setting levels.
I'll have a mess around with the motu today & let you know how i get on..
Thanks again!