"The nodes we have already have a 1 TB SSD (will be used for cache) per node (3 nodes)"
The first thing you should be doing here (if not done so already) is validating that the components you are planning on re-using/using are on the vSAN HCL for the intended purposes - e.g. SSDs used for Cache-Tier in Hybrid configuration should be certified for this purpose (and similar for Cache-Tier/Capacity-tier in All-flash). Do not use any devices not on the HCL if you care about your data and/or uptime as these are not tested nor certified by VMware (or they were tested and denied). How are the current SSDs attached? vSAN requires controllers to be used that are also certified for the release version (e.g. 6.5 U3, 6.7 U2/U3) and configuration type (e.g. Hybrid/All-Flash).
Certified components can be searched on the official pages directly or via tools such as this:
"Also how many 2 TB disks should we buy if we are aiming to have 10 TB of usable disk space. (using https://vsan.virtualappliances.eu/ it gave 5 - 2 TB disk per node ~ 30 TB of raw capacity). Node bays are 2.5"."
Yes, you would have ~10TB usable allowing for the advised overhead with and FTT=1 of all data with 30TB-32TB of raw capacity (but do note that disks sold as "2TB" by vendor are generally actually less by ~8-10%). I would advise not filling all bays if you plan to expand this in future by adding disks/Disk-Groups as opposed to replacing current disks with larger ones - but then again it depends on the nature of expansion e.g. if adding more VMs/increasing compute also, then expanding in future by adding more nodes is more beneficial for numerous reasons.