Good morning,
Mounting an SSD will speed up your Mac as ****, with no doubt.
On Amazon you can find cheapest SSD (Asenno, KingDiana and so on): for my personal iMac I will buy Corsair and Samsung, no issue and I've mounted many of them on MacBook and iMac.
For the guide you can refer on iFixit website: search iMac 2013 and then you should see all the reaper's guide, including switching to SSD.
If you want to keep the original 1 TB HD then you should mount your SSD replacing the DVD burner, always on Amazon there's several kits to do this.
Remember to install on SSD macOS and software, for collecting data the 1TB drive is perfect.
Best regards Marco