Thanks for the suggestions guys. That mac mini at makro is R10k with only 4GB RAM with an HDD. For a dude without moola, I'm rather picky about what I want. Perhaps I should look at an upgradable older mac mini?
One question that hasn't been addressed: is there some adb equivalent that I can use for debugging from a windows/linux machine with a connected iOS device (iPhone/iPad)? My main concern is ease of debugging during development, and the cheapest way to achieve that.
As for IDEs, I've become an IntelliJ fanboy. I hope to be buying their full suite of products soon (work sponsors a premium personal license for IntelliJ, so I wanna do a deal with them where I pay the difference for the full suite).
Nope, you need a Mac. All interfacing works via Xcode & iTunes (to be clear you have to download them)
IntelliJ has support for both Objective-C and Swift on OSX, so continuing to use that won't be a problem, except you're going to have to learn to use the Xcode CLI and toolchain for the rest, basically IntelliJ's coverage is good on the editor part, but they naturally haven't duplicated the entire toolchain but they do however provide tie-ins to some of this. Also IntelliJ has added support for Swift on Linux, but in all honesty it's not yet as reliable as the Android you're used to.
As to developing iOS apps natively on Windows and Linux; this won't be possible because the frameworks are only available in OSX: compiled to both OSX X86-64 and iOS ARM64.
Remember it's not the job of IntelliJ, Microsoft or Canonical to build these; if any it's for Apple and there's no value for them?
Apple are however building the Swift Foundation frameworks to work on both Linux + OSX, and the open source community is busy porting this for Windows, Cygwin, etc. but that's only Foundation (threading, file access, etc.), iOS and OSX have a ton of frameworks and there's just no value in porting this + not everything's compatible.
Note: Apple will typically only port frameworks that they see a value in; the Linux value for them is on the backend, i.e. that excludes UIKit.
FYI Microsoft did start the
WinObjc project to ground up build a lot of these frameworks for Windows, but their goal is to allow iOS Objective-C apps to be compiled in VS to either run unmodified or easily be ported to Windows phone and tablet -- like Apple they're not interested in making it easy the other way around.
Finally: older mac mini is always a good low cost option; but remember on Macs you are limited to upgrading RAM and HDD, so pick a decent CPU. With the newer ones, even that has become a challenge (i.e. more Appliance like). Alternatively you could find an older iMac (better specs than mac mini)