Sync between iMacs of different vintage

I have an iMac Mid 2011 (IM1) and am about to buy a new iMac (IM2) but will keep IM1 for my secretary to use with my business. I’ve used DT for some years principally for my little law business and currently use DT3 on IM1 and intend copying it with all my databases to IM2 once I’ve bought it but so that my partner can access it I want to retain only the database relating to my business on IM1. My existing licence enables me to use 2 seats so I assume that that will be OK. Please tell me if it isn’t!
I would like to set up Sync just for that database. I have been reading about Sync in the latest Manual and Joe Frissell’s Take Control of DT3 which for a very low tech guy like me is rather daunting and I have a couple of questions
1 IM1 OS is limited to High Sierra and won’t be updated to a newer OS while IM2 will have the latest OS and will be updated. Will there be a problem in using Sync in these circumstances?
2 If it’s OK I will have IM1 and IM2 in the same location ie building but I don’t have a local network at present so far as I am aware. Is it possible to create one to use Sync in that way or do I have to use Sync with one of the other alternatives eg Bonjour/iCloud etc?

No. Except that older versions of macOS won’t be supported by DEVONthink endlessly.

All cloud sync locations require of course an internet connection on both Macs, Bonjour requires either a local network or a direct connection between the Macs.

Only local sync stores do not necessarily require a network and could e.g. be located on external drives but that’s actually cumbersome if multiple computers should use this drive.