This will be likely be the most expensive part of the upgrade. Largely due to cyclic fluctuating flash memory prices.
Want to buy SSD with DRAM for longevity. Unfortunately not so easy to determine which ones have DRAM. WD Blue, Samsung EVOs (see compatibility issues below), and Crucial MX series are my go to for DRAM SSDs. ebay used SSDs are a good cheaper source. Risk is how much wear they have (Most I've purchased have good health remaining) Put SSD inside extenal USB enclosure and check with DriveDx on macOS or CrystalDiskInfo on Windows.
Do note more poor flash memory makers from China are entering the market and can fail rapidly just outside the return window. Probably the major high quality brands are the msot trustworthy but also the most expensive. These include Samsung, Sandisk, Intel, WD, Crucial
Samsung SATA SSDs have known to have incompatibiities with some Apple Macs. If having problems and have Samsung SATA SSD, keep this in mind
https://forums.macrumors.com/threads/2011-imac-wont-mount-samsung-evo-850.1971588/post-32926269
Also read post #17
Here are additional experiences with 850 EVO 250GB
https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/post-33495854
If Mac has upgradable SODIMM memory slots. Check everymac.com for compatible speeds and can usually get 8GB for $10-$15 on ebay. Used SODIMMs are quite robust. I have yet to buy any used ones that have ever failed.
iMacs have ZIP (Zero Insertion Force) sockets to replace the CPU. It does require more challenging logic board removal and thermal past (Arctic MX-4 or similar, avoid liquid metal) Look at the higher end spec variants of the iMac for quad core CPU and those should be compatible. Check socket type and search online for example upgrades successes to be certain.
For example, iMac 11,3 i3-550 dual core can be replaced by i7-870 ( < $15 on ebay/aliexpress ) from the higher end 11,3