IANAE - Someone more knowledgeable feel free to correct me if i’m wrong with any of the following.
I think the rule of thumb is 2k passmark / transcode so that i7 3770 would get you 4-5 transcodes + probably however many direct streams you want as they tax the cpu very little. Your work server would add ~1 Transcode, and upgrading the cpu’s would get you ~1 more. These are all rough numbers and would depend on what else you are using the server for.
Your I7 is in the NK4 guide, that’s basically what I started with moving from a synology nas a few months ago. It worked well, but when some dockers would sporadically take up a lot of my processing power plex would suffer. So I upgraded to a quicksync box and haven’t had a single issue with plex since. Still ended up wanting some more umph and room to play so I decommissioned the nk4 and built an Anniversary 2.0.
If you don’t want to have to worry about transcoding power check out jdms quicksync guide and seperate it out from your storage. An 8th gen i3 can get you 15+ transcodes.
I don’t think I would do it any other way after trying out the quicksync method. You can get one of the qs boxes for $200-300. Well worth it IMO.