a) go for the latest engine within your budget
b) the 3 letter engine code is stamped into a tag on the left hand side of the head, take the plastic engine cover off to see it.
The AGU is not a stronger block, it just has a larger ported head than the other 1.8T engines. The only 20v's with stronger internals are found in audi TT 225's, S3's and some (late ones ?) Ibizas. From memory these have stronger rods and bigger wrist pins.
DBW = drive by wire, no throttle cable.
NDBW = .. well, work it out
c) AUM has VVT, so sure, go for it. AGU would be more the choice if you were going to go BIG turbo.
d) as far as i know its just different mapping in the ECU.