I really wouldn't be rushed to get Doedee signed. He's a medium/loose defender long term which is not a position that holds much (if any) value, and hasn't shown anything which suggests he can switch to a high value position. If he does show signs of being able to switch to one (midfield would be the only option due to lack of height) then it's a case of sign him ASAP.
Right now, it's most likely a case of us offering him 450-500k at the end of Doedee contract, and Doedee staying as that is close to the max he could earn in his current role (I'd have the absolute max at about 525-600k). Equally, if Geelong are willing to give us a 1st + 2nd, I'd be comfortable letting him go. He's very replaceable by virtue of the position he plays after all.
Personally I wouldn't wait till his contract is up for renewal. I would present him a new contract and with good money well before his renewal date. Let him know that we are offering him extra not because of his contract ending but because thats how we value him.