To upgrade or not and the path to go. First question, do you need WAAS, if yes then continue reading.
If you already have a Garmin GNS430 installed there are a few options available. Most economical is to upgrade the existing GNS430 for GNS430WAAS (about $4300 for the hardware upgrade) another 300 Euro for the LPV STC, about 1-3 hours replacing the existing GPS antenna Garmin GA56 for a Garmin GA35 (needs a new connector and possibly also a new antenna cable) and a few hours to compile the work pack.
Second option is an Avidyne IFD440 which will fit into the same tray and needs no installation. The antenna will need to be replaced but a free STC is available from Avidyne. You can sell the old GNS430 on the used market for a reasonably good price. The IFD440 is a modern touch screen unit with bluetooth and wifi connectivity (unlocks purchased separately).
Third option is the Garmin GTN650. This is the way to go specially if the instrument panel needs additional avionics such as an audiopanel, second COM/NAV and/or ADS-B transponder. Garmin has a wide range of free STC’s available so no additional installation approval applications will be necessary.