Which comes first? Dual drive, fan, battery

Just like a surgeon stabilize before you do something major ;-}

So I would start off replacing the battery its quite easy to do. Here's the IFIXIT guide you'll need to follow: MacBook Pro 15" Unibody Mid 2009 Battery Replacement. Then finish things by replacing the fan following this IFIXIT guide: MacBook Pro 15" Unibody Mid 2009 Right Fan Replacement. Do be careful lifting the fan connector as its quite easy to pull it off the logic board if your not careful. Both guides list the parts info.

At this point you could either go a bit deeper either upgrading your drive or swapping out your optical drive for a second drive. But before you go down this path, keep in mind your system can only go up to El Capitan OS-X 10.11.x. It can't support any of the newer features within MacOS Sierra.

So now you'll need to decide how much more to put into the system. Think about what you currently use the system for and if its capable to take care of you for another year or so. If you think you've likely need to upgrade to a more powerful system nows the time to sell your's. Otherwise, I would just go with a SSHD hybrid drive to get a speed bump. I would go with a Seagate SSHD You'll want one of these.