I have 7 speed RSX, 8 speed RSX brifters on a mix īn īmatch Cannondale, 8 speed 105, 8 speed sora and 9 speed Tiagra, as well as various other mix īnīmatches.
In my humble opinion they all work very well. Sora most definitely is not junk - itīs just underrated, and therefore fantastic value second hand.

The single most important factor in getting them all to work crisply is setting them up right, taking the time to make sure everything is 100%, that comes from experience (hard won in my case.....and learning every day)

I would recommend going for that 8 speed RSX gruppo, if itīs cheap, as it will be a great source of spares in the future. AFAIK the current Shimano 2200 brifters are compatible too if you donīt fancy the DT shifters.

Hope this helps,
Barchettaman