You would give 20 XP each time you smith something? Would make it fairly easy to level, especially early on.

As for creating custom items, have an anvil (like you said) have a conditional branch, depending on the level. If level 1, blah. Level 2, blah. Basically like you said. If you'll need certain items to craft something, put it in the conditional branch on the anvil.

It would look awesome if you add a smithing menu that has graphics of what you can/cannot craft, your level, and exp, with the menu opening at the anvil.

I'm probably stating the obvious, but I like to think it helps.

As for the fishing system, I had a rough system in place for my harvest moon clone rpg in 2k. I doubt I have it, but I'll look.