Welcome to the forum
I have used Nobles for the last three services and I have found their customer service to be great. They also deal with Litchfield so is you want any parts they can order them and have them fitted but they won't do any mapping.
For tuning use JM Imports which is Newcastle and I recently had an intercooler and a remap from them. Jurgen is a spot on and they can help you out if you want to go for a massive build or just a general service.
I have used Litchfield in the past but I find it just a bit of a drive
The two cars that you mentioned I would personally go for the 2011 model as it has lower millage as long as the car is in good condition and good service history. If you are planning on tuning it doesn't really matter which DBA model you go for but the 2015 will have the thunderbolt head lights