Your idea about installing a "radiator style condenser" is basically the same thing as Walkes is talking about. The difference is, both the indoor and outdoor components will then be new and will be provided by the same manufacturer (say Liebert or Trane) and the equipment will be specifically designed to work together. If you try to "bastardize" it as Walkes has said, it probably won't work. Your idea calls for a new air-cooled condenser or air-cooled condensing unit (compressor and condenser) anyway in addition to the refrigerant line sets and control and power wiring, so the only thing left to purchase is the indoor fan-coil portion (blower and dx coil).
Other items: I don't know the logistics/physical layout of your project, but normally there is a practical limit on how far you can run refrigerant lines, somewhere around 100 ft. So your roof location for the condenser/condensing unit may be too far anyway.
If the roof deck is right above the computer room drop ceiling, you could put a new packaged air-cooled heat pump type "roof-top unit" and simply duct down through the roof into the plenum space and reconnect to the air distribution system (assuming it was ducted).
If the Onwer really hasn't enough money for a new system, perhaps some sheet metal-field-fabricated drain pans beneath the water lines in the computer area will do the trick. If there's an opportunity to re-pipe the condenser water in this area, there are also double-containment pipe systems available where leak detection can be put in the interstitial space for early leak detection. This could cost too however depending on the pipe sizes involved.
