There's a reason proper Pulse-Oximeters cost a lot of money.
Define "a lot"? You can get them online for around £35 or so, and that's buying local - buying at source in China will probably be 1/5th of that, but you'd have to buy a container full of them.
I must look up how the oxygen part actually works. The finger heartbeat sensing I already did 35 years ago with a small bulb, an LDR and some electronics :).