When I'm doing UI usability I usually just pull out a real iPad–the simulator is just there for "this change looks about right", and I coalesce a couple of these changes together before running on a real device. I rely heavily on AutoLayout, so most basic UI work is done on the iPhone simulator anyways; I only switch to iPad when I'm testing specific features such as split-screen support.
By the way, battery life on virtual machines has gotten better, I can get around 3-4 hours with a Windows VM running and seven or so with a lightweight Linux one. It seems like Windows has a penchant for preventing the VM from idling which really kills battery life.
I want at least 32GB of RAM.
My ideal iMac setup - 5K, 32GB of RAM, 2TB fusion drive, 4.2Ghz i7 - is $3099
A similarly equipped MacBook Pro would be $3000 with 512GB of storage (that would be enough). But I would still want a 5K external display - adding another $1300.
The last time I used a MacBook Pro to run a Windows VM, the battery life was so bad, I stayed plugged in most of the time anyway.
But back to iPad development. Can you really get a good feel of the UI usability of an iPad with a scaled display? I haven’t done any iOS development before.