I am looking at changing the frame to a steel hardtail (cotic soul, orange p7, 456 Dialled prince Albert, all the usual suspects). As my budget only stretches to older versions should I be too concerned about rust?
No.
I have a 30-year-old British Eagle road frame in my shed which was shot blasted about 15 years ago and nothing done to it since. It only has surface rust.
Unless the frame in question has been regularly dunked in sea-water then rust isn't an issue.
Check the previous owner has had it waxoiled and garaged and you should be fine 😉
No rust forms on the inside where you can't see it (if it does at all)
Rust isn't a problem for bikes. hence you hear about many people finding really old frames lying about and have restored them in to perfectly working bikes.
My commuter is a very old tourer (someone said it was from the 50's!) that's 60 years of road salt, rain, paint chips and whatever else.
It's still fine.
My alumninium road bike has more corrosion!
It can be a problem, especially with an outside or winter bike. But it almost certainly won't be. Damaged paintwork is likely to surface rust over time though. My tange-tubed carrera is into its 22nd year of either constant daily all year use, or total abandonment in a wet shed, and sure I took a fair amount of surface rust off it when I resprayed but it's still sound. And not many people use frames for 20 years these days.
I've seen a few old steel frames rust through usually on the seat tube where the front mech is (including a Yeti ultimate 🙁 ), or on the chainstay.
But it takes some serious neglect to make it happen IMHO.
My 16 year old Kona Explosif has had much of the paint rubbed off during it's hard life but there is little sign of corrosion and certainly nothing structural.