This a bit old info, but towers for item battles are determined on moment you buy/get that item. Higher lvl of hero , harder item battles.
So if you get some upgradeable item when you just start game with 1st lvl Olaf, and get same item again when you`re 70th lvl - battles will be big difference.
In the end, if yu want easy upgraded item get it as soon as yhou find it in shop, finish quest with upgradeable item as reward. But if you want more exp and rage skill usage then wait untill you`re higher lvl and buy item/finish quest for reward.
Hope i helped
