I understand that there are already questions like this for normal Raichu, but what about its Alolan counterpart?
I think it would be best to Evolve Pikachu at Level 33 or Level 42 because Pikachu learns Discharge at Level 33 which is really useful move for Alolan Raichu if you can't get any other electric move or wait till Level 42 because that's when Pikachu can learn Thunderbolt because Alolan Raichu can benefit a lot from Thunderbolt because of it's 90 Power and 100 Accuracy + paired with Aloraichium Z, Alolan Raichu can activate its Z-move, Stocked Sparksurfer. Or at any level Before you Evolve Pikachu, go to Konikoni City and Talk to the Girl with the 3 Pikachu and she'll first give you Pikanium Z and if you talk her again, she'll teach your Pikachu Volt Tackle which is the Signature Move of the Pichu Evolutionary line. Note that You have to have a Pikachu when you talk to the lady.
Hope I helped :)
Any level is fine, to be honest.
Its best moves, Thunderbolt and Psychic are TMs, as are Light Screen and Thunder. For ingame, once you get Thunderbolt, you can evolve it.
Or, you could have it hold a Light Ball, if you manage to get one.