I would go for the fastest processor (with the most cores) you can afford.
Do you know which sample sets you are going to use?
An issue with machines like the MAC mini is that the can't readily be upgraded, so if you need to upgrade memory or ssd size you may have to change the whole machine.
When HW runs a virtual organs the sample set data for that organ must really fit into main memory; with 32GB memory, the maximum size sample set you could run will be around 30GB. Is this sufficient for the sample sets you know you want to use?
HW Sample sets come with a large quantity of data which is installed by the HW installer onto your hard drive (or SSD) . Over time you may need to use an external disk drive to hold some of this data.
Have you read the information about possible problems with some recent MAC machines? (As a HW PC user (though I have a MAC) I can't verify any of this but you perhaps should at least be aware of what is being said...) See http://forum.hauptwerk.com/viewtopic.php?f=16&t=17380&hilit=apple+T2