Tom,
Used Bendix King, Apollo, Or even Northstar if you can find it.
Like others have said the cost for installation is significant.
I installed a KLN-90B 10 years ago. At the time it had the most bells
and whistles out there and was the easiest to use. It was also the most
expensive. By today's standards it is dog meat. It has over 1000 hours
on it without a problem. Solid, reliable. You will also need a display
for annunciation's and a deviation indicator.
I chose an HSI switched between NAV 1 and the GPS. That lasted for 9
years and then I replaced it with and updated one.
Michelle

tom pettit tompetatpeak wrote:

I'm currently flying VFR in a vfr equipped plane. What would be the lowest
cost GPS based instrument system I could install and be legal for IFR
flying?

I looked through the last six months of this newsgroup, but didn't see a FAQ
or the answer to the above, so I hope it won't be something that was just
hashed out.

thanks,
tom pettit