I am not a sparky, and live in a different jurisdiction, so YMMV. On the flip side I had the interesting experience of running a commercial hothouse, and the current tenant still ropes me in to help him with anything with power flowing through it.
My advice would be the opposite of @manassehkatz-Moving2Codidact.
I'd prefer plastic over metal as it does not corrode and is much more likely to be weatherproof. Also, you mentioned that your system has a neutral wire - I would argue that especially when you have a neutral wired up you don't need a metal enclosure. (Our solenoid controllers are housed in plastic our nutrient control systems - which are ancient are housed in a metal box, but all the replacements are in plastic boxes - and as these things cost 2 arms and 2 legs I don't think its for price reasons)
I would suggest against hardwiring the circuit into the mains (but do make sure their is a GFCI on the mains, or if getting an electrician to do that - at least have it built into the plug point) - At least in my jurisdiction as soon as you hard wire something in to the mains you have either breached code - with the insurance problems that follow - or you need to get an electrician in - in which case you should let him do the wiring and advise authoratively - and get him to sign off on it.
You may want to check the voltage of your controller and systems. Most of the stuff I've seen runs on 24 volts DC, with a wall adaptor. Even in a commercial setup we plug things in to power strips. Of-course, although its still a wet area [ie moreso then a bathroom], its not exposed directly to the rain/weather. You might want to check your setup as I would not not want to be running 120/240 volts to solenoids.
On the other hand we do get plug-points for direct use outside (ie as a newish home - this is not growing related - and we live in a country where the rain falls sideways - ie vey high winds). Our outdoor power connections tend to be downward sloping where the plug fits in so as to reduce exposure to water ingress. [ All circuits in a residential house here need to be protected br RCD's, so the outdoor circuits are RCD protected ]
In the 10+ years I've had an outside plug I've never had a problem with spiders or the like getting in to my sockets Hell, if that could be an issue get some plastic socket covers from a baby store (or just make sure they are single plugs with the plug always in!).