Yea it's okay to evolve him late. You can evolve him when you're about to go for elite 4s. This guide might help you to know about base stat / nature / IVs.
If you evolve your char early , it'll be more powerful for sure but it will require more exp and if you evolve at high level you won't need to grind for exp much. Best will be anywhere between 80-100 for story purpose.