Was about to get a toy- then I did some research...

[JAZZCNC]

1. Partly related to neomorph's comment- in an attempt to keep things simple I've considered using fk/ff ball screw supports mounted directly on the end plates/gantry plates, but this would require the steppers to be mounted with spacers (in between the stepper and the other side of the plate). The other option would be to use bk/bf supports mounted some distance away (on the "inside") from the plates, leaving enough clearance for the steppers to be mounted straight on to the plates. Do both work equally well in practice?

Yes either works fine. Also consider attaching the motors to the screws with belts as it provides a few advatages. It Lowers resonance, allows a ratio and gives flexabilty in location so can turn motors inwards and keep width narrower.

2. This might be impossible to answer without more details, but I'll give it a go. I currently have planned about 150mm of Z travel which would make the gantry plates roughly 500mm in height as measured from the X (long) axis side mounted rails to the top (ie the plates would extend a further 100mm below the cutting bed). Does this seem OK or would it be advisable to give up some Z travel in favour of shorter gantry plates?

With out seeing your design then yes it's not easy to say but 150mm Z travel is standard on router machines I build and 500mm length plates is about right so your not far away.