Kodak came in and updated our 2017-18 Prinergy and InSite Dell servers to Windows 2019 back in Sept. 2021.
Then updated Prinergy to v9.5 and later InSite to v9.5.
We had to purchase the Windows 2019 upgrades ourselves with Kodak's guidance.

We are now running v10 of Prinergy and InSite with no problems.
All Mac clients are running Ventura.

In a few weeks we are replacing our 2017-18 original Kodak Dell servers as they are now 6 1/2 years old and are at EOL.
Looks like we will miss out on Kodak having those new servers running v11 as that will not be out until late May early June.
We opted to pass on the cloud and/or VM's.

I do this frequently from my Mac at home to various Windows Servers and PC's at work.
Although i dont do a lot of file sharing.
No problems as long as your company firewall allows it.

We have Palo Alto firewalls and use the PA GlobalProtect Clients for VPN access and it always works flawlessly and fast!

I would recommend starting from scratch on that new Mac, in lieu of a Migration from a Time Machine backup or Mac to Mac migration.
Reloading apps and settings etc. may take awhile but I think its worth it.
Just cleaner and you dont bring problems over from the old Mac to new one.
That said I have done it both ways without much of a problem.

Not yet....but i would give it a try before going through the exercise of downgrading to Ventura.

How to on making a bootable USB...after you download the latest Ventura installer.

https://support.apple.com/en-us/101578

Also these...I have used a couple they make it real easy BUT they are third party not apple approved sort of speak.

https://www.macworld.com/article/671308/how-to-create-a-bootable-usb-macos-installer.html

I remote in to my Mac at work all the time from home using Apples remote desktop.
It works flawlessly everytime.
We have a Palo Alto firewall and their VPN app called Global Protect is fantastic.
Never have an issue connecting to our work LAN from home.

If this guy is a mac expert there should be no reason why he cant make sure you get whatever macOS you want on the Macs he is selling to you.
It sounds like he is telling you that the Macs he is offering you will have Ventura loaded as the macOS.

Apple can make that a bit complicated if you buy new from them or a reseller.
If Sonoma is installed you will have to download the Ventura installer from apple.com.
Or i use this site for my apple OS downloads....very handy for messing around with old Macs etc.
https://mrmacintosh.com/

We have 6 BenQ Monitors.
All 27 inch.
SW271C
SW2700

Excellent mid to high level monitors meant for what we do in prepress and for designers and photography.
I would recommend also getting the software to calibrate.
It is usually available as a bundle with the monitor.
They come with shades too which we do use.

Plus you that 27inch display!!

When making these decisions I always factor in that in prepress we are almost always 9 - 12 months, (sometimes longer) behind on updating to the latest macOS on our Mac's.  Due to Kodak and Enfocus etc not being ready.
So even though the 2021 iMac may stop supporting the macOS that comes out in 2025 or 2026 you would still have another year of viability.

That is true...but still get 3 years out of it I would think.

It appears it probably is an 18 core 2.3GHZ.
For prepress work it may be more beneficial to have more GHZ than cores, but i am pretty sure you would be fine.

i.e. The old 3.33 or 3.46 6 CORE was much better than the 2.4 8core etc...for prepess anyway.

Ask him to send you the specs.

Is it a 27 inch?
8 Core....18 core does not sound right...but i could be wrong.
Storage amount?

We have 5 Apple/Mac workstations here.
I have the Mac Studio M1 Max / 64GB RAM / 1TB.
Our other 4 are all the exact same 2018 i7 Mac Mini's / 64GB RAM / 500GB SSD.
All with 27 inch BenQ or older NEC monitors.
All running Ventura... because we always have to wait months for Kodak to modify Prinergy for the latest Apple macOS.

I would look into getting the M1 or M2 Ultra for hopefully a lot less than an M3.
Maybe from Amazon or eBay or OWC.
I also would not hesitate to buy used from the right reseller....like OWC.
I rarely buy new... all the Mac Mini's we have were bought used for 1/2 the price and none of them have had a problem.
This also makes the owners here very happy!!  Just a little more work for me hunting them down on eBay or OWC.

I have had the Mac Studio M1 Max for a year and half and it has been flawless...however at times i wish i had gotten the Ultra for that extra power when working on some of the larger jobs we get here.
Also if you work from a server, as we do there is really no need for anything more than 1TB of storage in fact 500GB is more than enough.

We will keep the Mac Mini's for another year or so as they will run macOS Sonoma and I think that will be it for them.
Then it will be Mac Studio's for everyone.