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.

I looked at the spec sheet for dynasty 200. It will- TIG weld at 100 amps while drawing 20 amps from a 120 volt outlet. That is an outlet served by #12 wire. It will have a 20 amp breaker. TIG weld at 140 amps while drawing 31 amps from a 120 volt outlet. #10 wire 30amp breaker. Your garage now probably has outlets wired for either 15 or 20 amps (check the rating written on the breaker handle). Those are the highest outputs stated in the table for 120 volts although the graphs imply that somewhat more may be available. If the machine is fed 240 volts the max is 150 amps and the duty cycle rises from 40% to 60% at max (drawing 15.8 amps). Again the graphs imply some useful output above this. All above is single phase. I would think that you would want to provide a 220 volt 20 amp outlet to get the most possible from this machine. It is neat that it has useful capability from a rather normal 120 volt outlet. This is all taken from the spec sheet (PDF) downloaded from here http://www.millerwelds.com/pdf/spec_sheets/AD4-8.pdf
I didn't look at the other machines but will if you ask it.
Bill