Hi Paul, before i decided on the 500 I watched the promotional videos from Sportys Pilot shop and Aircraft Spruce.
They indicated that an external Antenna was not necessary, and so I had the 500 fitted in my Jab panel at a slightly offset angle whereby i could connect an external antenna if necessary.
Its been in the Panel now for 18 months and I have never had a drop out.
The 500 is what Garmin calls the entry level Aera, and after looking at the 500 Vs the 510, I didnt think the extra money justified the cost of the 510, for features more suited to the USA than Australia.
I am more than happy with the 500, no glare issues and easy to operate. Very easy to load flight plans and the touch screen system is intuiitive. I practiced at home in Simulator mode.
The external Antenna is only about $25 USD I think, and should be available from any good Pilot suppy shop, and I carry one as back up only, its never been needed.
My advice would be to have a look at Aircraft Spruce and Sportys Pilot shop. The one thing to make sure is that it comes with PACIFIC data base, but with the dollar dow, it may be quicker and about the same price to purchase locally from a Pilot shop in Australia.
Hope this helps.