actually, both M-Audio and RME used to make external cards... but if you're going to get an old used model it might happen that it can be used only on Win XP operating system.
moreover, if you're going to get ANY external card which connects on firewire interface, make sure your computer has a firewire controller based on Texas Instruments chipset or you'd be simply asking for problems.
But, to stay on the safe side, it'd be better to stick to USB external ones only.
As just told above, I own an old M-Audio Fast Track USB card... bought around year 2004, and it only works fine on Win 2000 or Win XP despite it has official drivers up to Win 7 system.... but this model isn't good for you because it doesn't have 2 line inputs (only 1 microphone and 1 instrument/line input, then it's suitable to record from microphone or guitar/bass)...
But, there was another old external M-Audio USB card, named Audiophile USB, with line inputs which should be 100% compatible with audiotester... but, again, it must run with Win XP system.
I believe these can be found for very cheap at now!
Regarding old external RME ones, I remember their Fireface 400 and Fireface 800 models which rely on firewire interface (not the easiest one to make it working properly) and it was a quite expensive model).
That said, I doubt you need to spend as much as 500 euros to get a modern USB card with suitable line inputs...
I am seeing that, i.e., a Focusrite Scarlett 4i4 (3rd gen) USB card should be around 200 euros or so...
It has 4 analog inputs and 4 analog outputs... inputs 1 and 2 are with inbuilt mic/instrument preamp and maybe they can also get line level signal but they have gain input knobs which would force you to tweak/calibrate the gain just anytime you go and make measurements... while, inputs 3 and 4 are simply line inputs with no gain control knobs and, for measurement purposes, it's more handy to rely on line inputs with fixed levels.
Just, i need to check if this card works with balanced, unbalanced (or both) signals...
With the kind of devices you're going to use there, you definitely need unbalanced mode.
On my (a bit old now) Echo Audiofire 4 external (firewire) card, inputs and outputs can work in both balanced and unbalanced modes... but, hey, this is a model which has poor compatibility with audiotester software then I was forced to switch to another card for measurement purposes... but it works great with music production software, which is the actual reason why I had bought it, after all.