Reactors are pretty fool proof to make, even though I haven't made one, you are using materials rated for much more pressure than you would ever put through them, using materials that see far more abuse for their regular use. They are also really easy to make and all the special tools you need are pretty cheap. Not encouraging you to make one but just stating it shouldn't be too much of a challenge, even for the not so mechanically inclined.
Most the off the shelf ones are the same as what you would make, more or less so you are just spending extra money on not making one. There are two designs, the "Rex Grigg" and the "Cerges" style. I have only heard bad things about one inline and I forget which it is (Maybe the ISTA one, not sure). I know it's one of the super cheap ones, I believe the plastic is thin and can break.
The main downside of a reactor is the size of it. For me, I couldn't fit one under my stand without compromising a lot of room and having an odd plumbing situation. If I add another canister, I will go with a reactor but I just can't fit one right now without some crazy looping going on.
As for diffusers, you either have a inline ceramic one that has a glass housing, or an atomic style one that is plastic, and requires adjustable working pressure. The inline glass ones are similar to a glass intake diffuser, just inline. The downside is that they are expensive and fragile. The atomic style are much cheaper but require a higher working pressure. I haven't seen cases of these breaking on their own but I have my own concerns with mine, the plastic is not thick and between the high working pressure seemingly lower quality, I will likely begin replacing mine every few years.
I know I am not giving a specific recommendation because they all have their pro's and con's. I would go with a Cerges style reactor if space wasn't an issue. You can DIY it really easily, or their are some for purchase with the same style and fittings so you can replace them if something breaks. If you want a diffuser or need the space, that's a bit tougher because the glass ones are up to 4-5x more expensive than the plastic atomic ones and though they likely last longer, they are far easier to break yourself, but likely more durable in operation. I would just go with 2 atomic style from GLA and have the ability to swap one out when cleaning for less money.