Being an ex dennerle user myself I can comment on the following:
1) their regulator is crap. The fault is not the regulator itself but the needle valve on it. Lets just say that the design "beggars belief". I sent the needle valve to one shop to show him just how bad - and pathetic - a design it was. I bought a no frills regulator from this shop and used the needle valve on my Dennerle regulator. I only like the Dennerle regulator for the body encasement and the fact that you can change the sealing rings. I don't know what their new regulator looks like, but I wouldn't care to know.
2) because of the needle valve design you CANNOT connect your solenoid between your regulator and its needle valve. My changing the needle valve let me do this.
3) You can use RO tubing for CO2 proof hose, and this means you don't have to spend money on the expensive dennerle CO2 proof hose. You can use silicone hose too but CO2 will vent off the plasticsers from it and harden it over time because silicone has "holes" in it.
4) the dennerle non return valve is useless, there are better non return valves out there that do the same job without requiring so much pressure
5) the dennerle bubble counter is basic but perfectly adequate for use
6) I'm not a fan of their CO reactors/diffusers. I use the Dupla external CO2 reactor which is not available now, a good replacement would be the aqua medic unit
7) the best solenoid valve is the aqua medic pulse unit. You pay more for this, but it's a wonderful piece of equipment since it only runs the energy required during switching, and standby is a very low energy draw. I have a brand new dennerle solenoid that I never used and I'm happy to sell to you for a small sum if you want it.