Forgive me if you already know all this, but here’s a mini tutorial:
ADSB-OUT (you send a signal out) is now required in all US airspace where a mode C transponder is required. ADSB-IN (you receive data) is not required, anywhere. Of course the IN data is what’s most useful to you.
There are two forms of OUT data, either a mode SES transponder transmitting on 1090 MHz, or a Universal Access Transceiver (UAT) transmitting on 978 MHz. If you have a UAT you must also have a mode C transponder, to operate in airspace where they are required. A mode S-ES transponder is both a transponder and an OUT device. You are only allowed to use one OUT device at any given time, so no one installs both. Plus/minus: most UATs can be configured to hide your real N number, if you don’t like Big Brother (or your neighbors, who can easily access the data with a computer) watching you. At this time mode SES transponders always send N numbers, although there is talk of changing that. Note either system broadcasts your real N number if you are in contact with ATC getting flight following, ifr, etc (anytime your squawk code is not 1200). Mode SES is mandatory above 18,000’, if that interests you. No foreign country uses UAT, if that’s a concern.
Adsb-IN is optional. Many UATs include it in their box, most transponders do not. You may listen on one or both the frequencies mentioned. If you are within range of a ground station, it will relay to you the missing traffic (if you only can receive one frequency), as well as non-adsb aircraft detected by ATC radar. Ground stations will send you weather but only on 978 MHz. If you cannot receive that you won’t get wx. Now...
Your transponder will need a source of pressure altitude. Your ADSB-out device will need an ‘approved’ gps data source. [edit: your adsb-out must also use the same pressure altitude source as your transponder.] You will need some way of displaying IN traffic and wx data (panel mounted screen, iPad via wi-fi, etc). Whatever you choose, you need to verify that everything talks to everything it’s hooked up to. For example a Garmin WAAS navigator is an approved gps source. But it outputs the required data in a propriatary Garmin format, ‘ADSB+’. You need to verify that whatever OUT box you use can read that data. Bottom line here, you need to consider your entire panel, the whole system.
My recommendations: if you want all-new, it’s a no-brainer. A Trig mode SES transponder for under $2K, a compatible encoder (almost any modern efis, or a stand-alone device), and a $500 approved gps source from GRT. (If your new aircraft has a 430W the Trig can read and use that. May need Garmin software upgrade if it’s old enough, about $100). For IN (optional), there are many choices under $1K but you need to consider what display you will use, and are they compatible.
Complication. If you are willing to buy used equipment, the equation changes. So many owners have pulled good mode C transponders out and replaced them with a mode SES, that the used market is depressed. You can buy a good, used GTX-327 mode C for $400. At that price, a used mode C and a new UAT are competitive, and some cheaper, than my SES recommendation above. Of course this route involves more installation labor (if someone is being paid) and you need two antennas. But often IN is included in the package. And sometimes the gps is included too. So look at your whole panel, decide what you want/need/can live with, go from there.