Reading between the lines, I think that the mfr misunderstood what you were doing...I believe they think that you want to put the condenser in the garage (evaporator in the house or somewhere other than the garage). Ignoring the code (for a just moment), there is no technical reason why you can't cool this space with a mini-split. And certainly, such a unit will cool it more efficiently than a window (or through the wall) A/C unit.
But as steveray said, you are wanting to turn an unconditioned space into a conditioned space. The codes are concerned with energy consumption of buildings and as such, require conditioned spaces to be built in a certain way to limit energy use. Insulating walls, ceiling and floors is a big part however, limiting air infiltration/ex-filtration (leakage) is a also a large part. The key problem in meeting the energy code requirements will be the vehicle garage door as it doesn't have sufficient insulation and certainly, can't meet the leakage requirements. In other words, you won't begin the meet the code unless you remove the garage door and replace it with wall construction. Then, the insulation values in the wall and ceiling would need verified (remove all of the gypsum board on the walls for inspection and get into the attic to check depths of insulation. (In Corona, CA, below slab insulation is not required?) Install vapor barrier on walls and seal the heck out of everything. Have a blower door test run and check to make sure the leakage values are within spec. Intall new drywall and finish. Ta-Da! Conditioned space. NOW, have a mechanical contractor run cooling load analysis (and don't forget about telling him about the servers that are generating heat to the space.) ONLY THEN, can the proper size of HVAC system be chosen for that space. And you don't want to oversize a unit as short cycling is hard on the equipment and doesn't result in proper de-humidification of the air. Excess moisture within buildings can cause significant problems. Of course, EVERYTHING I have talked about requires one or more permits, even if YOU, as the homeowner, is the one doing the work. As cda indicated, it is strongly advised that ant electrical work be performed by a qualified and licensed electrical (who has pulled there own permit for the work). There are so many nuances to electrical wiring installation that is is not worth taking a risk on this part of the work. Sizing conductors is only part of the challenge.
In conclusion, you come to this forum (of mainly code officials) asking for DIY advice. I am (we are) not going to be too helpful towards a DIYer who wants to do something like this on the QT. The building codes department only exists to protect the public (you and the future owners of you home) from improper, perhaps dangerous, construction. Your taxes mostly pay for this service (the permit fees are only a small slice of the costs.) Let's put it another way: Suppose you purchased another house and someone (like yourself) did some work on the QT. The work was improper resulting in you having to spend money to make it correct. You wouldn't be too happy, right? Please don't try to circumvent the codes and enforcement that are put in place for good reason. The code officials on this forum have seen it all...done incorrectly.
