Closet grow.. air intake suggestions

I'm planning on doing my first grow in an upstairs closet later this year, so far the only thing I'm unsure about is getting fresh air into it.

My current ideas are:

• Aquarium pump pulling air from outside. I don't think it's going to be powerful enough to pull air from 2m of hose
  
• 100mm duct + fan. Ideally use a large fan and a reducer

Right now I'm thinking duct + fan. Though I'd prefer not to make a hole that size in the ceiling, it's not really an issue.
Has anyone got any other suggestions?

Thanks Figgy, though I should've said I'm in UK. Can't get than one here.
Didn't think to look for proper duct fans since they're normally expensive. So far I'm not finding anything cheap that's actually any good. Might just have to use a decent computer fan.