Ideally, you'd have a separate avionics master switch that you would use to apply power to the avionics after start; I'm a little surprised that you didn't have this feature installed st the same time as the 540. That said, modern avionics are a lot more resilient to spikes on the power rail and I seem to recall that the 540 manual specifically states that you can leave it on during engine start. Note that this may be undesirable for other reasons such as battery drain.
Charts wise, you will need a Jeppessen NavData subscription for Europe that will give you all of the IFR capability that you need, including airways, SIDs, STARs and approaches. This can be augmented with optional georeferenced charts for the various countries, e.g., I have the NavData + UK/France georeferenced charts installed as well as a world-wide obstacle database. Update cycle is as per AIRAC, i.e., every 28 days, although Jeppessen updates the georeferenced charts every 14 days. Subscriptions are usually annual.
I haven't bothered with any VFR charts although there is a subscription option for VFR, but I don't know what's included in that; if you find out, do let us know.
As for traffic display, you will need either full TCAS or an ADSB-in receiver connected to the 540 in order to display traffic.
Iceman
You are still dangerous! You can be my wingman
anytime.