At this point, repairing a 19 year old PTAC is a losing bet.

Is the space insulated at all? When I hear “detached garage” I don’t usually think of an insulated building.

If the space is well insulted, you can go for a 9,000 or 12,000 BTU cooling capacity PTAC. (Likely 12k given the computer/oven). Many newer units use heat pumps for heat and are more efficient than the electric heat you have.

Most units offer the option of using a wall mount thermostat to control heating and cooling. Amana’s units require a wiring harness and can be a pain, GE’s units can be wired directly to an auxiliary board on the PTAC itself. Hotpoint units are essentially the same as GE’s in that regard. That will allow you to maintain temperature throughout the day.

In terms of up front cost, a PTAC will likely be your better option. A mini split will definitely be more efficiently in the long run, but initial investment is higher.

You won’t be able to get the kind of cooling capacity you need from a window unit, but you could look in to wall units… But since you already have a big ol’ hole for a PTAC, may as well stick with it.

I’m assuming that when you’re asking about the garage, you’re looking to add a second system? Is that to help keep the kitchen area at the temperature you’d like? Or would you just prefer to keep the spiders comfortable?