Look up DTPS's web site. http://dptsassociation.homestead.com/homepage.html
It has lots of useful info. I do take exception with some of their info on places to spot however (it's a little dated).
I've reproduced below the illustration of the spotting places. Forget spots 1, 5b and 5c. The terrace has high, sctatched up plexiglass all around, and 5b and c are hidden by high brush. Spot 7 I think is pretty useless as well. Spots 5a you need a ladder except in winter, when the grass is not in your way. spot 9 you need a ladder. Spot 8 is a new parking lot (still green on the picture) belonging to TAG Aviation. During the week, you might get kicked out, and definately don't want to park there, but on the weekends there is no problem, including parking. You don't really need a ladder ther, but it's better if you did. you don't need ladders anywhere else.
Considering that none of the spots that are good in the morning are any good in the afternoon, you will need to move around by car - there are no convenient bus routes. Parking right next to, or at the most within 5 minutes walk from a spot is not a problem for any of the spots.
Don't expect any problems with security, as long as you don't start cutting holes (I caught someone doing that recently at spot 8).
When would you be coming? If I'm in town I'd be glad to show you around. I'll be in Vienna myself in a couple of weeks - don't know yet if I'll have time to get a little spotting done.
The only thing you should feel when shooting a terrorist: Recoil.