a) Do I need a Garmin bike computer - yes. Cadence, HR, Power and other cool stuff like Strava - I'm told KOM hunting is very addictive (apparently).
b) if so...why and which one..... there are 10 to choose from on the website. Whats compatable to a Q power meter - somebody? Anybody? I have Garmin Edge 810. Had it for a few years now and very happy with it. I'm not a tech geek though so there may be better models available by now.
c) do I need to buy 2 (one for each bike) - No, just buy one bike computer and set up a bike profile for each bike which you can flick between depending on which bike you're on.
d) how do you install them as the attachment thingy is for round handle bars not flat aero bars - buy a mount for each bike. Mine is a K-edge Garmin TT mount like this:
The device fits really snug between the bars. The gap between my bars is 55mm including bar tape so make sure you have at least that much space as I barely have a mm to spare as you can see on the left side of my Garmin Edge 810:
If you race with an aero bottle up on your bars, make sure you can still fit the bottle mount on without messing with the device position. I can do this with my Bontrager components. The device is almost invisible (see below). If you can't do this, there are other types of mounts that fit to various aero bottle systems but it all gets a bit stacked and messy.
e) If i get one etc, who sets it up and who sets up what power i am supposed to push and how do you find that out. - you set it up. It's easy peasy. Just go to Garmin connect and open an account for yourself, pair it with your bluetooth gadgets, HR strap, strava etc. Setting up the device is easy. Someone else will know about pairing it with your power meter but it paired easily with my Tacx Neo power meter seamlessly.