" I suspect it might be require a lot of components and cables to hook everything together, so it might be less convenient than a standalone recorder, but I'm curious about some of the details."
depends on the setup. in many cases it is more complex and less fool-proof than a standalone recorder. the primary function of a phone is not to run your recording app and its doing any number of other things at a given time, where dedicated recorders are single purpose and rarely crash/reboot/fail to do their job
"I know I'd need a portable Class Compliant interface with a mic preamp."
yes it must be class-compliant, but does not need to have a preamp in all cases. your particular setup might be already powering the mics and feeding it a line signal. its important to distinguish what a "preamp" does. fundamentally it amplifies a mic-level signal to line level for recording at proper level. that said there are a zillion preamps from multi-thousand dollar dedicated boxes, to a simple analog gain corcuit or digital algorithim in the recording device itself. when you say to "need to have a preamp", mostpeople would take it one of two ways:
-you need something with a gain stage that can make a decent recording with mic level signals
-you need a device to power the mics as well
the latter is technically a "power supply", which are often integrated into what we consider to be a preamp. to complicate things, not all mics take the same power. most common are mics which require phantom power (typically 48V, sometimes called P48, but can be as low as 12-24V). jsut about any microphone that has an XLR connection wants (or will work with) P48, it is the standard
examples of class compliant interfaces that phantom power mics as well as provide gain would be something like motu M2, any of the scarlett usb mic pres, rme babyface, apogee duet, sonosax sx-m2d2, etc.
while these are all great units they are usually designed for other purposes (home studios, etc) and are pretty bulky relative to say a zoom F3, centrance mixerface, or other stand alone recorders and add points of failure between the phone itself and the extra cabling in between. an intermittent analog connection in a rig might cause noise. an intermittent connetion between a phone and recorder might sink your whole recording
to complicate things, not all microphones require phantom power. some use lower voltages like 2-5V ("bias voltage" often called 'plug-in-power (PIP) ) and this ranges from $5 electrets up to top-shelf small condensers like the DPA series which are designed to run with either P48 mic bodies, or bodies which use 5V bias power. these mics can often be adapted to run on P48 via off-the shelf adapters
examples of class-compliant devices which provide bias power: sonosax sx-m2, dpa d:vice
examples of standalone devices which provide bias power: most handheld recorders (though the 2-3V is too low for recording loud concerts with most mics), recorders and transmitters from zaxcom/lectrosonics/deity like lectrosonics SPDR, deity BP-TRX and (hopefully soon) pr2. Probably some otehrs i am forgetting.
but thats if your mic needs power. if you have a separate power supply/preamp you can also record to a phone using line in, such as with the rode AI-micro, or dpa-dvice with modified cabling.. i would hesitate to recommend a phone setup to take line in as there are so many small recorders such as the sony A10 that are smaller, more reliable, and have longer battery life than a phone
"Presumably I'd also need a power bank to power the interface.
I think I'd also need an Apple Camera to USB adaptor, but are there different versions of this? I read someone saying they'd bought the wrong one and needed one with power."
not necessarily. depends on the interface. larger ones that provide P48 like a motu M2 need a camera adapter to power the unit from an external source. an iphone with one simple cable connection can power a d:vice running mics for 7-10+hour and there is no camera adapter or external power needed
sonosax sx-m2d2 doesn't "need" battery power but i don't know how long it lasts in real world situations. today its common to bring a power bank into a show so no big deal with security
"What about software - I'd just need a mono or stereo recorder. I've seen the Hindenburg Pro app mentioned, but that might do more editing than I'd need. "
apogee metarecorder as mentioned
"Where would the audio files be created and saved - directly on the iOS device or somewhere on the Cloud?"
on the device initially. as mentioned, metarecorder can send to dropbox right after the show if you dont mind burning 2GB on your data
"Finally, how would I get WAV files of the recording into my Windows DAW?"
4 ways:
1. via itunes
2. via third party app like copytrans filey
3. via external usb stick like a sandisk ixpand
4. get on wifi> send to dropbox>download
hope that helps