Used Avionics

I plan to buy my first airplane and "trade-up" in about 3-4 years. I
expect my initial investment will be around $75K. At that price, it
does not seem to be worth putting in brand new avionics to the tune of
$12-$15K (thinking specifically about a Garmin 430/MX-20 combo, or a
GX-50/MX-20 combo). At the sametime, I really would like the
situational awareness benefits of such avionics. Is it practical to
consider buying used avionics? If so, where might I get used avionics
(web site pointers, phone numbers, or email addresses would be helpful
in addition to names of places).

By the way, thanks for all the great help I have been getting on this
forum. It really helps me make some hard decisions about my first purchase.

-Sami