Thank you all, very helpful stuff. I think I will go for the 'cellular' iPad Mini and see how I get on. I am assuming 16Gb will handle everything? I don't use the iPad for storing photos etc, all of that goes straight to Google Photos so I am assuming 16Gb will suffice?
If you don't load huge amounts of garbage on the iPad, the smallest memory version will do the job. You only need large memory versions in case you have the addictions to tons of music or videos, or are unable to delete things.
I use both, Jeppesen MFDVFR and Skydemon in parallel on an iPad and plenty of space available, even though it is also my main businessPad with the MS Office suite etc on board. Memory usage is not dramatic; JeppMFDVFR: total 3.0 GB; Skydemon: 1.1 GB, and both have lot of countries, VAC and AIP loaded.
Yes, a 16GB Mini will do the job just fine and if you go cellular you have all options for GPS, internal or external (and I suspect in 95% of the aircraft you don't need an external - focus on deciding with respect to need, not status). The cellular will also enable you to do a lot of things while on travel, such as filing delays for your FPL, or have a look at current weather, or do a vacation trip with daily flight planning on the iPad ... etc. If your flying is i.e. density altitude sensitive, you will find things like the AutoDens.app very useful - and experience is vastly improved with working internet connection. I also advice to stick a SIM card in there even if never used, as the first GPS fix is usually a lot faster with the assisted "a" of the A-GPS.
One more experience, I started using an original 9,7" iPad and found it a slightly too big in the cockpit - or better, I thought I needed it mounted somehow to always have a look at it. I then switched to a Mini, which I believe is still the best solution for crowded cockpits and was happy with it for quite a while. But, after a while I realized that the use of the permanently mounted iPad degrades my biological navigation ability, because I was sucked into Magenta Line and follow the blue line addiction. The same time the magnificent iPad Pro was presented to public in the original 9.7" form factor and I went for that. My feeling - the Apple Pencil with the 9.7" iPad Pro is probably the best solution in the cockpit, at least for me.