In my personal opinion, it's more important to get the hot air out of the garage instantly.
Aye. In-Rack AC with external radiator and putting that outside the garage might be an efficient option. Installation is not that bad - insulated tubing + power + condensation management. However you might have to check if the external parts can handle the environment, i.e. sub-freezing in my case, dunno if that ever applies to you.
Climatized racks are also nice for operating gear in dusty/humid(&salty) areas, as the sealed environment keeps all that out. So if you're looking to buy used, don't limit yourself to data center sources but look also at industrial. Low power in-rack coolers are used to cool control cabinets and might be good enough for your application.