ADS-B in is available in one compact unit- PowerFLARM portable. Alternatively, PowerFLARM core can be fitted behind a compact screen on the panel. And it picks up Mode A/C/S transponders and the thousands of gliders in Europe transmitting FLARM as well.
In the long run, I think that affordable ADS-B in/out is the best solution but PowerFLARM, despite not being cheap, was definitely the best game in town a couple of years ago when I got it, and probably still is. This is because it was/is designed as a collision avoidance aid and gives an audible warning of collision threats, as well as the less useful virtual radar display of other devices.
PS I have no commercial connection to FLARM or PowerFLARM.