- What equipment does the aircraft have to have to be able to do this?
If you're intending to operate only outside controlled airspace, then not much from a legal standpoint. You need a turn and slip indicator and an altimeter.
From a practical standpoint, you want at least the basic instruments, at least one reliable radio, and whatever you need to fly the approaches at your usual haunt
and alternate. I would suggest at minimum a VOR/ILS and ADF. I'd also suggest investing in a good handheld GPS, and test it in the aircraft.
To fly IFR in class D/E airspace you need VOR/ADF, transponder (C) and a COM radio. To fly in class A you need VOR/ADF/DME, transponder and a second altimeter.
- If an aircraft is advertised as IFR equipped, what does that mean? That you can fly airways with an IR? Or just that its got VOR/DME/ADF?
It means only what the owner intended it to mean, which could be as little as turn indicator and altimeter!
- do I need FM immune radios to be able to legally fly in IMC?
If you legally require a nav radio, then it must be FM immune.