I’m only being asked about 1 entrance door. This is a small business (3-6 employees).
The most basic "single door" entry systems typically don't have an audit trail or the ability to send notifications (this would be things like the Securitron DK-26SS). You'll need something a bit more advanced. How are you recording the cameras, nvr or what VMS?
What kind of door hardware / door is it? A picture of the door could be helpful.
Some possibilities, there are others. You'd have to add a magnetic door strike or electro-mechanical lock.
Unifi Access G2 Starter Kit Pro $600 and CloudKey+ $200
The non-pro kit is $300 cheaper but, codes to enter aren't an option have to use fobs, cards, or phone app instead; no video intercom on non-pro.
ICT Protege WX (seems like a nice system, but not sure how hard it is to get your hand on)
DMP X1 Single Door Access control or XR550 if you want a more capable system (alarm system with access control) (I think this has a subscription cost)
https://www.dmp.com/products/access-control/x1/x1
CDVI A22 $683 and CDVI + Keypad reader or A22KITSKB $1103
Brivo ACS-SDC / ACS100 (less secure as relay / sensitive wires on outside of building) / ACS300 with B-BSKF-B Keypad reader (These may have a subscription cost)
ProDataKey Red 1 $454 or RedCloudNode + Keypad reader (These have a subscription cost)
Kantech KT-1 + Keypad reader
Dahua ASC3202B / ASC2202C-S + keypad reader
Cansec Webster Mini + keypad reader (verify keypads are supported)
Be careful with some of the basic non-subscription type systems about your network topology and don't forward ports or do anything dumb like that (particularly the last 3 in this list).