Wow, a lot of examples, but seems we may have gotten away from the OP's goals.
There is plastic track that will stand up outdoors, it's flex track from Train-Li, look them up.
The plastic Lionel track will not hold up outside, but if it is cheapest, you COULD paint it, but what a pain.
Used aluminum flex track would probably be the next least expensive choice.
Yes, you can buy inexpensive battery powered locos, and they will run for a while, but a quality loco will be less expensive in the long run, like one from USA Trains, like a small switcher, etc.
For simplicity, battery power and an inexpensive remote control like from G Scale Graphics can work.
You CAN get into the hobby inexpensively.
Greg