Pokemon-go – Which Pokémon have a further evolution that isn’t yet available

Any Pokémon you don't have, is worth more than repeated evolutions. So trying to complete your pokédex is always a good idea for gathering experience.

After that, you should primarily be concerned with the following three evolutions:

Weedle > Kakuna
Caterpie > Metapod
Pidgey > Pidgeotto

These evolutions cost only 12 candy, while all others cost at least 25. Since all evolutions reward you 500 xp, these three give the biggest bang for your buck.

And if you can, use Lucky Egg to double all xp you get from these evolutions!

Yes. Gyarados is accessible in the wild.

As it does not have a rarity class¹ (note the PokemonClass on Mew or Mewtwo) assigned to it, it may spawn freely (albeit rarely) across the world.

However, due to their spawn chances, it may not be as much work to evolve a Magikarp -- unless you're exceedingly lucky and near one of the biomes in the game that supports Gyarados.

^{1The data sourced in the above dump came from packet and game reverse-engineering and has since been validated by many people, including myself. It is perhaps, as of now, the best source of information we have on what makes Pokemon tick.}