Ahh Mike, you've got a fun time ahead of you.
For imaging point sources, aperture is more important, for extended objects then f ratio is more critical. So if you consider a widefield shot as basically an extended object then the faster the lense the better. Others might argue th point that a widefield is a collection of pointsources then so be it, but from my limited exprience I've found the faster the better, and then stopped down a half stop or two.
Focusing is fun
especially with the zoom lenses as the infinity setting doesn't really focus at infinity
you do need to back off a bit. (that and finding someway to keep the zoom ring from moving when your focusing. I use a lump of blu-tak to hold it in place) Check out the image here on John Drummonds site for focusing using longer lenses. For me focusing through the view finder is very hit and miss. My astigmatism makes the view look closely focused but when I take a test shot it can be way out. Hence my dependance on DSLR Focus. Test shots and viewing on the LCD does help if your doing it manually, but I'm an automation junkie
http://www.possumobservatory.co.nz/a...5-focusing.htm
Have you checked the accuracy of your platforms tracking Mike? It might be worthwhile downloading something like K3CCDTools V2 with the free monthly trial and using the drift explorer to get an idea of its accuracy. Can you manually guide your platform in RA? You're DEC is non-adjustable once your latitude is set isn't it? It may be a lot of mucking about but with the K3 you could do a drift align, certainly for your altitude ajustment (I'm assuming it has the capability of fine adjustment and not just notches for different latitudes), not sure how you would do it to get it fine enough for your Az ajustment though.
JM2CW