COOLING

My first suggestion is to make sure that you install an air filter for the fan. Without AC, the last thing you need is a lot of dust getting into the server chassis and blocking the air vents.

Have you considered installing a window AC unit?
Even if it exhausts into a different part of the building it can keep your server room cool enough.

If forced air is your only option, I would consider a dual fan approach. An air in fan mounted near the floor. (Perhaps a furnace blower) Then, on the opposite side of the room an exhaust fan mounted as high as possible.

Match the CFM on both fans. If you can't find equal CFM fans, have the inlet fan be the one with the extra capacity.

Chas

I would recommend using as much passive cooling as possible. If you already have AC piped into the server room then the best solution would be an exhaust fan pulling the air from the room. Ideally it should be in the ceiling to push as musch hot air a possible out.

As far what type of fan to use it would depend on the size of the room and the temperature in the room. Remember that larger fans that spin slower make less noise and push as much air as smaller fans that turn faster.