I was about 25 weeks when i got all the big stuff like cot,change table etc. and then got the rest in dribs and drabs! We picked up our pram about 4 weeks ago and our car seat a week ago - so it has been kind of a work in progress. Also if you are planning on laybying (sp?) things then you want to account for that time too and actually its not a bad idea. I bought it all outright so it was all sitting there doing nothing for quite a while!! Nice to look at though!
I actually set up the nursery properly (rather than things just lying around!!) at about 32 weeks and am still in there fiddling with bits and pieces, changing blankets around etc. nearly every day - but thats more from boredom and excitement i guess!! Good luck!