This. SSD and extra ram will certainly speed up you imac. I have an iMac similar year to yours, I've done many upgrades on mine over the years, which now more resembles a hackintosh more than the original iMac.
Changing the hard drive in iMac's are fairly easy, I've done it before. You'll need the right tools, which can be picked up very cheap, and there's lots of tutorials online to guide you step by step through the process. If you're not practical, or good with your hands, then take it to a computer specialist.