I had the same question a while ago, and I picked the brain of one the
local CFIs at my club about what he thinks is practically necessary and
what is just a perk for IFR flight. He said something along these
lines:

Airspeed
Vacuum attitude indicator
Sensitive altimeter
Electric turn coordinator
Vacuum heading indicator
Vertical speed indicator
Clock
Outside Air Temperature
Heated pitot tube
Alternate static air
Mode C Transponder
Dual NAV/COMMs
Moving map GPS with RAIM (probably with WAAS)
2 VOR indicators, 1 of which has a glide slope
Marker beacon

And only if you're feeling saucey:
DME
ADF

Also, you'll need engine and system monitoring instruments. Some of
these you can buy more than one in one box like the combo GPS/NAV/COMMs
and combo OAT/Clock. Also it might be worth looking into an electric
(no vaccum) panel with glass displays, but you should have redundant
electrical systems and back up airspeed and electric attitude indicator
outside of the primary flight display.