Is Sonex a good trainer plane?
I would avoid the Sonex, or any other kit aircraft, until you have some student flight time under your belt and are sure that's the way you want to go.

The problems with using a kit LSA to learn include:


The "Build then fly" process of a kit without a license assumes you will want to do the latter after doing the former. Plenty of novice pilots spend 5 to 10 hours at the yoke and either get it out of their system and lose interest, or wash out in other ways (fail medical, can't pass written tests or checkrides and give up, run out of money to spend on rentals and fuel). If that happens to you, you'll be stuck with a kit you bought new, which you can't fly and now have to sell used. It's better to rent your way through the first few weekends as a student, then once you have your student cert, a few flight hours, solo endorsement and you're sure you are able and want to keep going toward a sport or private cert, you can look for an aircraft to own and practice with.
Building a kit means getting that kit certified airworthy. An amateur-built plane has to go through a rigorous mechanical inspection before you'll be allowed to take it up in the air. The fact that it's a kit for an aircraft fitting the "light sport" type rating might make it a little easier, but they might treat it as an "experimental" craft just because it's a homebuilt and go through the "Special Airworthiness Certification" process for those types of planes.
Buying someone else's kit means inheriting their mistakes. If you don't build, and instead buy a kit plane that someone else went through the trouble of building and getting certified, you're still flying a homebuilt. Anything the FAA inspector missed could cause a failure in flight, and you can't just pull over and call a tow truck when that happens. The same could be said of your own homebuilt, and overall with the strict mechanical inspection criteria of planes this isn't as big a problem buying a plane versus buying a used car, but it's something to think about; they built and configured it their way, and if that's not "your" way or even the "usual" way it can be distracting in flight, and distracting is dangerous.
The Light Sport class is designed for day VFR with a limited range. If all you want is a sport pilot certificate to go out flying in fair weather near your home airstrip, great, grab an LSA like the Sonex and have fun. If you want to progress to a PPL, and fly cross-country, at night or in IMC with your instrument rating, you'll need a plane capable of flying in such conditions for your checkride and other training time including at least one serious cross-country flight. The stock Sonex builds I'm looking at now do not have the IFR "big 6" and really no room to mount them. You can go glass, but instrumentation's only one piece of the puzzle; you'd also need an external light kit which doesn't come stock, and the airframe has to be rated for flight in minimum winds as well as IMC like rain, which most LSAs simply are not designed for.
LSAs have limited power by design. That doesn't mean other aircraft don't have similar power ratings (the stock Cessna 150 is only 100hp while there's an engine kit for the Sonex providing 120), but the Sport Pilot license was designed for flying LSAs with a max power of 87hp, with an instructor endorsement required to fly anything more powerful. Just one more hurdle to getting rated to fly your own up-engined Sonex (or anything else besides).


Just my analysis. Again, if you just want to start with a SPL (20 hours flight time, some ground instruction and a written test, no medical beyond the basics for a driver license) and see where it goes from there, then a Sonex kit, base price about 18 grand, is a pretty good deal assuming you're good with tools. But if you want to progress to where flying is useful beyond simple recreation, you'll start rubbing up against the limitations of an LSA sooner than you would in a more full-featured trainer. That more full-featured trainer is also going to be more expensive, but it'll be a plane you can use much further into your flying career, and if you take care of it, will hold cash value and even match inflation so you can use it as a store of value to trade up (the 150's original retail price back in the 60s was around \$7500; those same '60s airframes change hands at around \$25,000 now).