Great choice of projector! I just got my SP5700 yesterday and so far it is fantastic! Even Sky digital through the RGB scart adaptor looks great. Far better than the HS10 I had before. Only negative point is that the colour wheel seems to be quite noisy on mine - makes a high pitched whine. I'd be interested to know how loud yours is when you get it.
Regarding positioning, I would also advise you to mount the pj on the ceiling; the 5700 has an image offset of 33%, so you are likely to need to sit the pj low on your trolley. This combined with the relatively long throw needed means you are likely to have things in the way.
I also was looking for a fixed screen, but they were all a bit pricey. In the end I made a fixed screen for mine using mdf / blackout material and it works very well. However, I think the pj would benefit from a grey screen and may get some material from harkness hall in the future. Have a look on their site -
http://www.harknesshall.com/ - they do a grey material. Someone more knowledgeable than myself may be able to help more about screens.