At the risk of propagating a discussion in which the wires may already be substantially crossed, for European IFR you need to meet the respective national requirements for IFR, which can be found in the AIPs as I wrote earlier.
In practice this means VOR, DME, ADF, ILS, BRNAV GPS (panel mounted obviously). Plus a second altimeter and other stuff I don't remember. I wouldn't fly IFR/airways with less, because if I got turned over on the ground it would be really obvious. It's a bit like flying an NDB approach without an ADF; I've seen good arguments that it's probably not illegal in Class G but that is hugely restrictive in the UK and useless abroad.
Mode S is also mandatory; concessions or no concessions you will have to fit it anyway pretty soon. That's £3000 or so.
To be 100% legal there is a mind-bending list of trivia and one could get awfully anally retentive about the exact wording of the POH supplement for the BRNAV GPS for example. Almost nobody understands this stuff which is just as well.
A lot of planes simply cannot take this equipment. Not enough panel space, or you could do it with a completely new panel but that's a very expensive exercise (which avionics shops really love).
Finally, remember that secondhand avionics fetch almost nothing. I sold a good-as-new KT76C Mode C (with all the documentation) on U.S. Ebay for US$1300 but that won't work for anything old.