You'd get a pretty big boost in CPU perf going from a dual core i3 to a quad core i5. That core i5 2600s is a pretty reasonably priced upgrade. Going from i5 to i7 probably averages about 10% more performance, so not great value for money for your upgrade scenario.
Do you have 1x8GB RAM module, 2x4GB or 1x8GB? You should be able to go to 16GB RAM if you're only at 8GB now.
You are, more or less, "stuck with SATA" unless you replace the mobo, ram and CPU.
(There are a few ways you could add a PCIe SSD as a non-bootable drive but they're not worth pursuing). Going to a bigger SATA SSD will give you more fast storage space and better performance than your current SSD. I had (and loved) a C300 128GB as my first SSD but the market has come a long way since then. You can get a Samsung 860 EVO for about $80 or go up to 1TB for about $150. These are great all around drives and could be transplanted to a newer build down the road. Make sure the SSD is plugged in to one of the 6Gbps ports on your motherboard (specs say there are 2) as modern SATA SSDs are able to saturate the port at peak throughput.
Samsung 860 Evo
If you were doing a new build, I'd probably suggest an NVME drive for even faster throughput, but the gap in user experience between an NVME and a SATA SSD is nowhere near as big as the feel of a SATA SSD system vs an HDD system.
So, $30 for CPU upgrade, $80 for new SSD, $30-60 to upgrade to 16GB RAM and you'd have a nice little upgrade and a system that feels quite a bit snappier. Total $110-170
Here's a quick value build parts list for an overhaul. Ryzen 2200G (4 core w/ onboard gfx), motherboard, 16GB RAM and a 512 GB NVME drive
Case, power supply and secondary drive could be reused. Total about $340
https://pcpartpicker.com/list/HchvyX