A suprising amount of stuff is EASA approved (either directly under EASA or by a previous Euro CAA). If you stick to this, that's the simplest way by far. This is true of much of the JPI range.
Anything else is going to rely on avionics shop teeth sucking and their interpretations of regs which are, shall we say, somewhat variable
If you feel lucky, try getting a PRNAV approval while you are at it
Actually I am not kidding - don't put in any fancy avionics today unless it is PRNAV approval capable (basically that means having to use a GNS430/530 and preferably the W versions.