Hello! As far as i know there isn’t any Warden guild around for now, and the community which was around is no more aswell.
However, Warden RP is a tricky thing and may be hard, as Wardens are basically high ranking, and also their duty and motivations are special, but nothing denies you that you can’t do it.
It’s better to start as a Watcher, which is basically a lower rank one, however in lore Maiev’s wardens and Jarod’s Watchers have been separated a bit… Regardless, you can be both, yet Watcher will be easier and it’s my suggestion
And for a guild, i strongly recommend the great guild Moonlight Embrace, which is pro Kalimdor and Kaldorei, and led by a Warden and has Watchers in their ranks, a really good bunch and really recommended to join 
Not sure if other guilds would have Wardens/ watchers also, as Wardens specially, are something that can’t work for anything…
I think other night elf (military or not) guilds MIGHT take Watchers in though, like Nightblade sentinels, WEIRD Company and Hand of Cenarius… Forgive me if i missed any!
As for armor, there are several combinations that can work, but wouldn’t completely look like Maiev or Sira, except the Paragon toy… which needs lots of grind and you will need to keep it with prism, which makes you look like Legion Sira/ legion warden model
Sadly for now we don’t have Large cloaks yet, which Maiev has, so you may have some problem
But for set suggestions… Depends on your type of armor
For plate,i suggest WoD blacksmithing crafted one, or the legion invasion recolor if you have it
Also the HC (?) DK set HELM from Antorus which is green is really a cool helm for them, you can combine it with some sets
For mail, Gruul’s lair one (if i recall correctly) which a bit does look like one
For leather… I forgot which set, but the one Shandris uses (unrelated, just saying which one) and is from TBC raids…
And there are plenty of others to use aswell
And final suggestion, i suggest you to join the Kalimdor/ Night elf Discord community, which there you can establish yourself, or learn more and other things, feel free to poke me on discord for an invite: Lightbearer#8393
Sorry for long response!
Good luck and… Let the hunt begin