Just a guess but I suspect any make of helmet will melt under the conditions you've described in
THIS thread
Perhaps the glass of the porch intensified the suns rays causing the problem you've had? I've stored helmets in my plastic shed in the past and never seen anything like that.
FWIW my helmet is stored in it's protective bag in the bottom of my wardrobe, and funnily enough it's never melted either
Mind you it's not a Scott so that may be a factor in it's survival
As for a new helmet, personally I'd never buy a second hand one, so just see what offers the online places have got. If you know your size then you can't really go wrong, but if in doubt get to your LBS and try some on, then buy the one that fits best.
Oh and son't forget the safety standard shenanigans as well, Snell/EN 1078 and all that guff
