Depends what you mean by IFR flight.
If you just mean hand flying in IMC and doing some kind of procedural approach, then in the UK all you need is an altimeter and a turn / slip indicator or an AI and DI.
If you mean airways flight in CAS then realistically you need to be RNAV5 capable which to all intents and purposes requires a GPS or something like the KNS80 (but that's a bit of a joke really - especially as more and more VORs get shut down).
There are loads of other requirements for airways flight such as dual altimeters and 8.33 radio.