Easiest way to find what amperage is available to your garage is to see what size breaker is feeding the garage's sub-panel if you have one, or feeding the circuit you would be plugging the welder into. The main consideration is the wire size feeding the garage sub-panel/circuit and although unlikely, it is possible the wire size could warrant a larger breaker than is installed.
For a good size machine you would also want 240V not 120V. You can tell if you have this by seeing if the breaker supplying the garage is a double breaker rather than a single breaker. Naturally, a voltmeter is useful too.
I am not familiar with the specs of the TIG machines but like every other tool you have to determine what you want to do with it, either now or in the future, and then see what machine will handle the job. This at its simplest means knowing the materials and sizes you wish to weld.