Same as Vickster, I've an odd shaped, squareish head and many helmets are too circular for me so I wouldn't buy a new brand without trying one first. See if you can borrow or try one where possible. POC and Giro are the only known good fit makes on me, there might be others but I haven't found them yet. I've found a few that dug into my forehead however.
"Shelf life" is a pure marketing tool, whether a helmet needs replacing just comes down to the condition of it, whether it has it suffered impact, been cut, punctured, or yes, maybe even if it's just manky and offensive. A lightly used lid won't suddenly melt at the 5 year point but you did the right thing, you inspected it and decided it probably needed replacing.