You should verify what
infotainment / MCU you currently have (Software -> Additional Vehicle Information) as that will significantly impact how soon you might get FSD. Model S/X switched from Nvidia / MCU1 to Intel / MCU2 March 2018, and very few vehicles with MCU1 have gotten FSD Beta 10.69 early last year while those with MCU2 are getting the latest FSD Supervised 12.x.
Purchasing FSD Capability includes the cost of
service installing FSD Computer / HW3, and your cameras should be good as you already have HW2.5. If you do have MCU1, I believe a small number of people have gotten the upgrade for free while others are happy to pay for the more responsive / faster touchscreen experience, but if your primary desire is to use FSD Supervised, you could associate it to the whole cost (I believe an additional $1,750).
What vehicle software version are you on? That will also indicate whether you have MCU1 (2022.8.10.x) or MCU2 (2023.x+ or 2024.x+). And it could also affect what version of FSD you can update to in the near term if you do purchase.