Reynolds 500 = perfectly serviceable tubing - yet not a performance lightweight tubeset really. It'll most likely have serviceable components hanging from it, which may or may not be in great condition. If the bike is to serve its purpose - transport - and it's the right size, then if it's in OK condition it'll not be a bad bike. If it's in great condition it'll probably be a better bike (maybe with a bit of TLC) than a lower-end brand new Halfords jobbie.
If you're wanting a lightweight thing to take adventuring then probably look elsewhere. I'm not saying it wouldn't go off-road, but it'll be a bit of a lump.