Although I have never done 0 scale outdoors, I know or knew a number of folks that have; here are my thoughts:
1. Track: You need track that is UV proof if it has plastic ties. When it was first introduced, Atlas said its was, but whether that's still true I don't know. Peco says its track is, publishes specs, and has their own outdoor layout. Wooden ties such as Gargraves requires treatment, and even then renewal after x years.
Unless you are using battery power, choice of rail joiners is important; the old Atlas/Roco NS ones seem to work best. [ + multiple feeds, of course ]
2. You should give battery + radio a good look to eliminate track conductivity problems. This will also reduce the need for weatherizing + safety concerns with 120 vac outdoors.
3. I would avidly learn everything the "G scale" folks can tell you; after all, they do more outdoor rring in a day than all 0 scalers in a year -- see if there's a local [ well, in the state ] garden rwy club. I'd see if you could buy or borrow a big stack of older Garden Railways mags, too.
4. Plastic might melt -- but rust is a bigger problem for metal.
5. Is your property vandal proof ? Even the original LG&B was heavily vandalized by neighborhood kiddies. Do you have squirrels -- some of the more technically inclined ones seem to like wiring to chew on; hence, conduit.
6. Turnouts: Group them into "interlockings" and cover them when not in use, even if they are hand thrown.
7. Is your back in good shape ? If not, do you have any teenagers for forced labor ? "Drainage, drainage,...." -- just like the prototype.
8. Think big. But, unless you're a dedicated bachelor, get "Management" fully on board, and maybe even involved.
9. Most importantly: Have fun ! [ and show us your progress too ]
Best, SZ
Edited to correct typing erors