The typical ST head THD is pretty high from waveform distortions (ratcheting from non-skewed rotor windings, harmonic hump from harmonic winding) will have no adverse affect on your welder. All the new designs are switch mode power supplies, so the first thing they do is create bulk DC from the AC input. The cheaper welders with bad power factor (direct rectification of the mains to bulk capacitors may do this in a manner that has lousy power factor. That will affect an electronic regulator, if you use one on your ST-7, and it is a typical cheapy that doesn't do true RMS voltage regulation...causing it to regulate voltage too low. I've experienced this situation on my ST-3 with electronic regulation. On your stock harmonic winding, it will regulate fairly well with a rude, low PF load. i switchover my ST-3 to the backup harmonic for this situation.
The usual 10 Hz Listerflicker variation in voltage and frequency will normally be handled quite well by a decent electronic welder.
The big issue I see you having is that you will have to limit your welding current pretty seriously. If it's a small MIG welder, with output at 24VDC at the machine, you should get 85-90 amps of welding current (allowing for losses), but you're at the limits of the 6/1 power (2300W generated power) If you can live with that, you should be OK. I have had a smaller amp MIG welder running on 230VAC, on my 6/1-ST-3 with harmonic regulation. It was fine for thinwall square tube welding.
For moderate current stick or other higher power welding, you don't have the power for 1/8 rods of 7018. You could barely do 5/32 6013 rods. If your duty cycle was low enough, and you had SOM type heavy flywheels, that would probably help. An 8/1 would be a better match for home/farm welding.
PS I've got some heavy flywheels I'd sell at cost if you get serious about doing an 8/1 conversion with an aluminum piston and have the typical 6/1 spoke flywheels. I don't think anyone thinks they should be spun higher than 650rpm.
Best Wishes,
Bruce