The rust doesn't look too bad to me. I've bought worse looking frame sets than that off ebay, although clearly I'm not a rust or engineering expert so only you can say if it's worth the risk or not to you...
You’re probably right. Most looks fine. It’s just a couple of shots of the rear BB shell (and tubes around it) on the purple frame that look a bit odd.
Edit: My potential stoker has pulled out also. So I think I'll pass on them. Shame, they're only 20 miles away down the A1!
https://www.ebay.co.uk/itm/274604880897
I’m suspicious of some of the rust on these. But if they’re structurally sound, then...