Upgrading Refurbished iMac Need A Shopping List

I am looking to possible purchase a refurbished iMac. The information on the iMac is below:

• Mid 2009 iMac Apple part # MC015LL/A
• 2.0 GHz Intelcore 2 Duo Processor
• 2GB 1066 Mhz DDR2 memory - supports up to 8 GB
• 160 GB 7200 RPM hard drive
• NVIDIA GEFORCE 9400M
• 256 MB GDDR3 memory shared

I know I want to upgrade the hard drive with 2TB not sure if I should do Seagate or the other one listed. I also want to get 4GB RAM not sure if I should just get the one or two. If I get these items I would need the Spudger, Suction cups, TR8, TR10, Phillip screwdriver that's it, right?

I am trying to see what the best cost either I upgrade the refurbished iMac or I purchase an iMac that is already upgraded

(Info of one someone is selling: $500 Mid 2011 model, The iMac has a fresh El Capitan image on it and all old data has been wiped from it. It comes with a keyboard and mouse as shown in the picture, it has 8GB of RAM, 1 TB of storage) ???