Sounds like you don't really need much advice
One issue which might arise if you have no experience of projectors is the DLP "rainbow effect" - though it affects a minority of people badly, many people who do see it aren't particularly bothered by it or find it only a minor irritation, and even more don't see it at all or only very occasionally.
Try to get a demo of a HD70 if possible.
LCD projectors don't have this potential issue, though argubaly don't perform quite as well as DLP either (though these days, generally speaking, the difference seems marginal to me) - however the LCD v DLP debate is an old one which is still raging on, so you'd have to make your own mind up on that.
The other issue is image size - at 4m it seems the HD70 will just about be able to do your screen size, but at 5m it can't throw an image small enough (from projectorcentral's calculator the image from 5m distance will be 3.0-3.6m diagonal, while from 4m distance it'd be 2.4-2.9m diagonal - a 2.5m diagonal image can be obtained at a distance between 3.4-4.1m approx)
Viewing distance shouldn't really be any closer than about 1.5x screen width (or you'll start to see the pixel structure), which, for a 2.5m diagonal screen, would mean you should'nt really sit any closer than around 3.3m away from the screen - though this is always down to personal preference in the end.