8 BG is too little for After Effects. 24 GB minimum for good performance, preferably as much as you can fit and afford.
As fast a processor as you can afford is best for Premiere Pro and After Effects.
Anything that can run Premiere Pro and after Effects well will run all of the other applications well.
Depends on the media you want to work with in Premiere/Speedgrade. However the Dual Core CPU in the mini will limit you more than the 8GB of ram. 8GB of ram would be minimum.
thanks for all your help