Sunfish,
I think you will find that the transponder is the one thing that must be TSO'd , and any GPS source for ADS-B out must also meet all the necessary specs.
As for things like altimeters, ASI etc, buy overhauled TSO's from the US (or here if you can find a competitive price) because many of the el cheapo stuff will not meet the calibration rules.
Tootle pip!!
PS: I strongly recommend "needle and drum" altimeter, as opposed to two/three needles altimeter.