HI Craig

Personally, I wouldn't go as low as a 6mm EP - at least not to start with.

Low focal length eyepieces - high magnification and unless the condition of the atmosphere - known as "seeing" are very good, you'll find that you don't use it much (if at all).

Most important accessory to get first (IMHO) is an external 12V power pack (not a mains adpter) such as a PowerTank or an Auto Jump Start battery.

Next would be a Barlow lens (although I gather that the ETX 80 has a built-in Barlow).

Then an EP around 18-20mm and, if funds stretch, another around 10-12mm.

Some people go for a Zoom EP (say a Baader Hyperion Click-Stop Zoom) which covers focal lengths from 8-24mm and costs only just ove the cost of two decent fixed-length eyepieces.

Others may disagree with my first few "purchases" and I'm sure that if they do, they'll advise you shortly.

Hope this helps

Art