12V fan questions

[jl10011]

It seems like some people are using the 3 1/8 Coralife fan, which i am afraid might be a bit too big for my 18 x 18 x 27 orchidarium (both visually and air movement wise).
Perhaps 1 fan like this might just do the job?
Would I need two fans to get thorough air circulation?
Could I get a voltage adjuster to slow this fan down?

I was also thinking of going with 2 smaller fans I think...
I've found some smaller 40mm and 50mm 12 volt fans.
My question is what are people using for their ac/dc adapter/converters?
Are any of these adjustable?