Hi, and welcome to the forum!
I'm kind of in your same boat, and waiting for parts to complete my first LED build. I can't answer everything you're asking, but I might be able to answer some stuff until the pros get here.
First of all, have you seen LEDG's post on example strip designs? There's one for 3' x 3', and I myself am trusting his judgement on my 4' x 4' LT-FB24B build, as I don't see trying to reinvent the wheel and am yielding to one who has exponentially more knowledge on the subject than do I. That said, your skill-set is different than mine, so what's right for me might not be right for you. You've probably read it (and maybe watched his videos), but here's the link:
http://ledgardener.com/diy-led-strip-bu ... bridgelux/
I believe "B" type potentiometers should allow you to dim to @ 10%-100%, unless there's some issue with going below the CC driver's voltage range should you dim below 50% (again, I'm still learning this stuff myself...so please wait for one of the, "Pros from Dover" to chime in).
Installing eighteen 22 inch strips into a 3' x 3' area is going to be a job, and I don't see any viable option that excludes you needing to mount some vertically and horizontally on the frame's plane. It'll require @ $80 (shipped) of 1-inch heatsink (
www.heatsinkusa.com) to do it right, or you could spend a little less using aluminum C-channel...but I opted to do mine using proper 2-inch heatsink (I'm using the wider Samsung LT-F24B 4-foot strips x 4).
Running that many strips in series MAY cause a voltage dropoff towards the end of your string, where the front strips will be brighter than those towards the end...IIRC (again, wait for the pros to chime in). If this is the case, you may need to wire everything up in parallel, or a combination of parallel/series. If you check out LEDG's build design using 15 of your strips, they are wired in parallel (5 groups of 3 strips each). However you wire it, I know LEDG suggests 18 gauge wire from Wago to strip and strip to strip (I think 18 is the thickest wire the strips' connectors can accept) and thicker, 14 gauge wire from Wago to Wago, for better safety. (But there's the boiler-plate disclaimer:
Consult with a qualified electrician before assuming any info here as being more than just a casual guide!)
As far as the spectrum goes, either 3000k or 3500k was suggested to me as a very good all-around spectrum, with this grow-pro kind of leaning towards 3000k. After doing a bunch of research, I opted for the 3000k (as my space will be primarily used for flowering). That said, I've seen several stellar grows using 3500k. I did also get some 5000ks for a veg space I'm constructing.
So as a beginner just getting my feet wet, I'm deferring to the expertise of the site's administrator to design my first light; I trust he knows way more about this stuff than I do. But again, you may have a different skill set, so go with what you feel. There's more than one way to skin a cat, but until I get more knowledge, I will follow the example build design.
Good-luck, be safe, and again, welcome to the forum; it's a good one.
J.R.R.
Oh yea, forgot to add; wiring all strips in parallel. Voltage droop is a real problem with this many devices in a series circuit. All those little bits of inefficiency stack up by the end of the road.