In due course my aircraft will get North America ready with the necessary ADSB conspicuity requirements. Not sure what the Canadians will require, so for the time being looking only at the USA ADSB ‘OUT’ requirement.
I do not have WAAS GPS so my options appear to be:
1. Upgrade to a Garmin GTX 345 all in one ADSB OUT transponder
2. Upgrade either the 530 or 430 to a WAAS OHC unit and upgrade 330 to 330-ES
3. Install a UAvionix tail beacon presumably with the upgrade of the 330 to a 330-ES
My inclination would be 1 as the cleanest standard, but 3 is probably the cheapest?
On a similar vein why does European airspace want to know whether you are WAAS equipped, and does it give you any special access privileges in routing?
IIUC it’s for performance based navigation.
Canada uses space based ADS-B via 1090ES. Right now, only Class A airspace requires it (18000 MSL+). You either need to have a top antenna (diversity) or be able to be received adequately by the satellite systems. Next May, the requirement will add Class B airspace (12500 to 18000 MSL) and eventually the lower E airspace will get added in the 2026 timeframe. uAvionix has a 1090ES transponder option https://uavionix.com/products/tailbeaconx/
If I were going to upgrade, I would install the GTX345D if you want to fly in Canada sometime in the future and cover the US. It provides ADS-B In that will work anywhere and in the US it would provide both weather and traffic.
Where does UK/EU airspace require you specify SBAS? It is permitted to be specified, but I don’t think it has any practical usage at this point. It can be specified in the US also, but is dutifully ignored. In field 18, SUR is used to indicate compliance with US and Canadian ADS-B Out mandates. For US, one can specify SUR/260B or SUR/282B or both and for Canadian mandate, you specify SUR/CANMANDATE. If you comply with both, you would specify SUR/260B CANMANDATE. These codes are in addition to 10b where you specify both a transponder and an ADS-B Out code. For the most part, specifying the codes in the US have little effect, but there are some circumstances where leaving them out might result in a route being assigned that does not depend on ADS-B Out.
It is SBAS, known over here as EGNOS. WAAS in the US. You need the transponder to be fed with an SBAS GPS in order to radiate a certified (SIL=2) ADSB-OUT signal.
You don’t need SBAS for PBN. You need it for LPV only. You don’t need it for +V (the “synthetic glideslope” thing) which is just as well in light of this.
I was also looking at the GTX345 to replace my 2005 GTX330 but never got around to it, and in Europe these fish are just not worth frying. From conspicuity POV there is almost no IFR GA in Europe and airliners use Mode C for TCAS, not ADS-B.
Mode C for TCAS
You mean Mode S.
Airliners use your Mode C (they disregard your Mode S) for a TCAS warning (TCAS-1).
For airliner-airliner RA (resolution advisory, TCAS-2) they use some Mode S based signalling whereby the two transponders (the two TCAS-2 systems) negotiate between themselves which one will tell his pilot to climb and the other one will tell his pilot to descend.
No light GA implements TCAS-2 so the only relevant system to carry (for avoiding a crash with a big jet) is good old Mode C. Mode S just enables the UK CAA to bust you much more easily, by broadcasting your tail # 
Nothing airborne can see any of your Mode S data. Exception: Avidyne TAS605 connected to an old Avidyne PFD via RS232 (not ARINC429) and then the PFD displays tail numbers, N-reg only. @Cobalt will remember.
I may have the above wrong but I’ve spoken to an awful lot of people about it and I am sure if it is wrong, wigglyamp will be here within the hour 
It’s not about Mode S data but about the type of transponder. I believe basic Mode C transponder won’t respond to TCAS interrogation.
It deffo does – I see them all day. TCAS-1 is just that. It pings nearby Mode C/S boxes.
Adsb is seen by all of the UK GA carry-on/Temp installed devices. Pilot aware and Sky echo.
It’s for thos reason I upgraded my 330 to a 330ES.
I still do a lot of VFR and I want to be seen by anyone try to ‘see me’
There are still a lot that don’t bother but it’s getting better.
@RobertL18C if as you say you’re interested primarily in ADS-B OUT to meet the US requirement in the limited airspace areas where it applies, a GTX 335 (versus 345) is designed for that purpose and can be purchased for installation by any mechanic, unlike the more costly 345 which has ADS-B IN too but by Garmin policy is sold and installed only by their dealers, who are also expensive. At least this was the situation in 2019 when I went through a similar review of options and settled on the 335. The result was much lower installed cost to meet the US airspace requirement, with some other benefits as follows.
My choice in the 335 was also driven by an interest in having as much of the equipment in the plane not permanently installed as possible and thereby easily upgradeable when the technology changes, as it always does. I also wanted to avoid Garmin Bluetooth in favor of reliable Wi-Fi for transmitting ADS-B information in the cockpit to multiple Foreflight equipped tablets and phone. With those factors in mind I use a Stratus 2S for ADS-B IN instead. Despite being easily removable, with some care it ended up as a clean installation that all starts itself with the master switch. I do appreciate having traffic info very much.
The Tail Beacon would work fine for your purposes too, and is super easy to install. But in my case the old King transponder and its encoder were on their last legs so it was efficient to replace them both with a 335 having internal GPS and integrated GAE 12 encoder options. I did have to install a Garmin GPS antenna but with remote encoder removed, no external GPS and no installed ADS-B OUT I have as little officially installed complexity in the plane as is possible, just one box and one antenna doing only what’s legally required for what will probably be a very long time. Along those lines the GTX with its antenna can also serve as a WAAS position input for some other avionics if needed, so one may not end up with multiple antennas all over the plane.
