Check to make sure your applications are compatible.
El Capitan 10.11 Compatibility information
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.
I have purchased RAM several times and a hard drive from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I had 6GB installed on an early 2008 iMac supposedly limited to 4 GB and noticed an improvement.
Other World Computing