I have also been looking at Tablets solely for SD and have drawn the conclusion that for now anyway I'll stick with my Tesco Hudl II which does a fabulous job and has never let me down. It's only problem is that it's no longer supported and sooner or later it will not run modern apps. They are still available on eBay.
As I am primarily looking at Android over Apple my options seem to be the Samsung S3 8" which is the same size as the iPad mini (so your mounts should work) or an old Nexus 7. There is a nice Huawei 5 8" which seems ideal until you read that you can't run apps from the SD card.
Everything I've read about running iPad's for SD seems to be hassle. From overheating to not having built in GPS people still rate them and I'm sure their fine if you are prepared to put up with that but for me I'll stick with Android.