I don't even know if that is the right way to ask this question or not but here is what I am looking to do. I have two airplanes and neither have transponders. First plane is an Aventura 2 Amphib. This obviously isn't a cross country machine and I don't fly into any airspace where a transponder is required. One exception to that is Mode C veil - I would really like to be able to fly into Detroit and Chicago's mode C veils. I would like to get basically the cheapest used transponder I can find that will allow me in a few years to ad ADS-B out if I decided to. Second plane is a Quickie Q200. I just picked this up as a sort of project. It has 230 hours on it but hadn't been flown in a while and needed a bit of a facelift. The PO never had any electronics - not even lights or a transceiver. This is obviously a cross country machine so I definitely need the transponde. I again want to get into a inexpensive used transponder and I will be adding ADS-B out in a year or two. The issue is as I am looking at used transponders it is very hard to determine which ones will be compatible with ADS-B out. Looking for any advice on hoe to take care of this without braking the bank.