Ok, firstly, motorcycle helmets age from the date they are first worn rather than the date of manufacture (so don't buy an ex-display one and expect it's full life to be intact, it will have been tried on many, many times)...many lids are months old by the time they get to the UK, then they may spend several months at a dealers...so if it's a helmet that's 2 years old, it doesn't have 3 years left...it has 5. Only wearing them really begins the ageing process, but common sense would say to be wary of anything that dates back to the mid 90's !!
Adding aftermarket stickers and having them sprayed, is a bad idea...even the best masking could in theory allow some solvent to get to the polystyrene inner, causing it to deteriorate. Stickers can react with the shell bonding agents causing weak spots. Saying that...small stickers can be added to fins as long as they are not a part of the actual shell.
For a good fit, the helmet should not be too tight as it will cause headaches, and not too loose. One trick is to fasten the strap and pull up the back of the helmet, making sure it doesn't pull up too far. A nice even pressure all over is good. A new helmet will bed in somewhat, so a slightly tight helmet is going to be a good fit after a couple of weeks.
Dropping a lid is not the end of the world...unless it's packed with your daily shopping, then the things inside can dint the protective inner. Check it over...light scratches etc are not too much of a worry, although an obvious crack would mean commom sense tells you to replace it. Crashing a lid means it's bin worthy.