First off, I'd agree that it would be worth seeking guidance from whatever preserved railway it is about what they consider acceptable - unless you particularly want to get certain items for your collection, there might be little point in getting kit that won't be approved of.
From a personal perspective, I'd be inclined to think that BR uniform (now 20 years' obsolete - history didn't stop at a particular date) might not be entirely out of place on a heritage railway, especially if that railway does have 'rail blue' era / livery rolling stock, in the same way that it would not be inappropriate for bus crews at a 'running day' type event to wear pre 1985 National Bus Company kit / badges. (there's probably a photo or two out there of me driving an NBC liveried bus, and wearing one of those bloody awful octagonal hats they issued)
And for that matter, there's already buses in preservation from the post-deregulation / privatisation era, and another decade or two on there may be trains to match.
However, it might look a bit odd if (for example) different members of station staff at the same station were wearing pre-grouping, big four, British Railways and British Rail era uniforms, likewise if one individual was wearing a mix of several eras' kit - although I understand some long serving railway staff took a pride in wearing some pre-nationalisation badges or uniforms in to the 1960s.
And at the risk of possibly stating the obvious, you refer to 'red logo' - I take it I'm reading this right, and you're aware that (if memory serves me right) for a guard, you want jacket with blue rather than red stripes - see left picture on second row
here - i'm fairly sure that the red stripe was for 'railman' (station staff), blue stripe was for guards (think they had an option of a more formal jacket), and that there was a version with gold coloured trim (not sure if this grade had the option of a zip jacket) for senior station staff.
Similarly, if you're going to see what online auction sites have to offer, some sellers have very little clue about what they are selling - sometimes this means you can bag a bargain because an item's been so badly described, but it can also mean that all kinds of claims are made for how old / rare an item is...