It really depends on you. Synology is more plug and play solution which is easy to configure with the most basic applications.
If you want to save some money or get better hardware than used in Synology for the same prize than DIY NAS should be your choice. It will take some time to configure it, however, it is more customizable with a better hardware.
Check on the deals for the switch, it might be some especially on the upcoming Black Friday and Cyber Monday.
Also, you would need shared storage for the cluster. For the common OS, the best option for 2-3 nodes is starwinds vsan, it also doesn't require witness node for 2-node configuration, so you might save some bucks on it. It has minimal hardware requirements so, just put the drives and you are in.
I see, than good luck with finding good deals on the hardware!
Well, than, you should bge fine with VMUG subscription to get licenses. The options you have mentioned, should be able to run it.
You may get additional server and run 2 node cluster with Starwinds vsan. It is hardware agnostic, so, basically, hardware won't be an issue, also it doesn't require witness node which might save you from getting 3rd computer. I believe it is the most decent option for 2-3 node setups.
Hmm, obviously not...
Well, it depends from the worload, but I would really change them to new hardware already.
That is what I am talking about, they are still good for running them especially for SMB, if they can get refurb servers with hardware support.
As far as I remeber, it was possible to prolongue dell support for them, so, yeah, they are still good.
For the shared storage, you may try to go with the Starwinds vsan, the system requirements are pretty low and you would be able to use the internal storage as shared one, so no need of the additional box. Also, it has guide with the steps, so shouldn't be an issue: https://www.starwindsoftware.com/resource-library/starwind-virtual-san-vsan-free-configuration-guide-for-proxmox-virtual-environment-ve-kvm-vsan-deployed-as-a-controller-virtual-machine-cvm/
r740s are still fine to be used for production, even with new 15 and 16 gens available.
It depends on your use case, if the idea is simply to get the shared storage, the windows option should work there.
I would avoid Storage Spaces since it is too unreliable, especially the parity option. As alternative to it you might use stablebit drivepool with snapraid or collect the drives into software raid inside of linux vm. Shouldn't be an issue with Starwinds cvm https://www.starwindsoftware.com/blog/file-share-with-starwind-vsan