I have a older style Garmin 530. Can I use this GPS as the position source for an ADS-B installation?
In the USA, ADS-B requires a WAAS position source. A Garmin 530W can be used, but not a 530.
Here's a full list of FAA certified equipment needed for ADS-B. I hope that helps.
You'll need a WAAS GPS source if you need to comply with the ADS-B mandate but there are quite a few GA pilots who only fly VFR in regions where the ADS-B mandate does not apply.
It is permissable to use a non WAAS GPS source with your transponder if you wish to voluntarily transmit ADS-B in airspace where the mandate does not apply. The FAA and EASA seem to have gone out of their way to conceal that fact.