A lot of CO2 is DIY and most folks experience is going to be with systems they've rigged together themselves after prowling Beer Distributors, Welding Shops, and Lowe's/Home Depots. The AquaMedic regulator is the only "aquarium" thing I bought. Compact, works well, comes wih a decent needle valve.
Bob Alston's page has a lot of really good info and is probably as close as you'll find to a buyer's guide for some of this stuff -- valves, tanks, regulators.
The reactors are their own kettle of fish. I'm in the middle of assembling one now that's based on the Tom Barr DIY model and which vaguely resembles the Buschke -- same general principle. This seems to me to be the reactor of choice among the serious plant folk. The DIY model is a waste of time and money unless you like to do DIY. For all my trouble I might have saved $15 -- not worth the two trips to Lowes, never mind the pile of parts in my living room. But I like DIY

. Otherwise I'd get the Buschke or write to Tom offboard -- I think he makes/used to make these for about $30.
I looked briefly into the membrane difusers and it seemed to me they were somewhat limited in what they were able to handle and needed a steady flow of water over them to operate efficiently. Not really what I was looking for.
I always liked the spirally thing if your going to put something in your tank. But I want to keep the tank as hardware free as possible.
Briefly tried airstones and thought they were completely useless.
Getting decent results running the CO2 through my HOB and am getting used to the little brapppp the impellor makes every time a bubble hits it. But the uber-reactor is going online just as soon as I can find that one drill bit.