Most any USB interface with two matching inputs will be suitable, don't cheap out on an interface that has one mic input and one instrument input. I wouldn't recommend Focusrite due to driver issues under Windows, constant buffer under-runs regardless of settings.
For a low cost option there's Behringer UMC202HD, don't expect very high performance from it though for distortion/noise analysis, but for basic frequency response and impedance measurements it will be fine. I used to use a Steinberg UR22mkii which I would recommend as a solid mid-range unit, I can only imagine the newer UR22C is quite similar.
For a higher performance but still affordable option, Motu M2 or M4 are great. I use an M4 myself. Unlike the budget options like Behringer, Focusrite, Steinberg, Motu has a real balanced output, and both TRS and RCA connectors are available which is convenient for connection to most home audio equipment without needing a TRS to RCA adapter. The visual level indicators are handy, as a downside, the analog knobs for adjusting gain levels are digitized, they move in discrete steps despite the analog feel of the knob.
I use this jig in my setup:
Here's a schematic for a simple dual channel measurement jig, which can be used with ARTA, REW, or SoundEasy.
www.htguide.com
For Motu M2/M4, I'd recommend using around 1k and 8k ohm resistors instead of the 47k/22k shown for the voltage dividers.
