So, before you get your nose to the grind stone on building the right infrastructure… figure out what the right infrastructure is for your company.
Sure active directory is an easy tool to manage logons, policies etc. Does your business need that overhead? There is a cost to setting up AD, servers, Licence cals, etc. Before you go down that road, figure out what you are trying to accomplish.
You said your are a google centric company already for email, well you can use google to control PC logins. If you are just looking to control basic permissions, file access,etc. you can do that with google as your lead. some details here: Overview: Enhanced desktop security for Windows - Google Workspace Admin Help
You can set up a Linux Samba host to manage your logins etc: An appliance you can play with for it here: Domain Controller | TurnKey GNU/Linux
You can set up Virtual domain Inside AWS for a reasonable cost.
That’s logins, about the rest of the environment. What do you have on site for hardware, networking etc. Start documenting everything you have before you start on what’s next. Get spread sheets of PC’s Laptops, mobile devices. Draw up a network diagram marking physical locations, trunks, IP’s, access points, everything. Create list of all the software that’s used, what you have for licenses, who owns the billing, make lists of contact info for every technology vendor you deal with.
Spiceworks Helpdesk and asset management is actually an awesome tool to get you started. Get it installed on EVERY workstation, it will report back hardware and software details for all the machines. Use the network monitor to do the same for your network infrastructure.
DOCUMENT EVERYTHING!
Does your company have any established policies such as acceptable use?
Now that you have a good understanding of what you have, start looking at what your company does? what is it that IT needs to provide the company as minimum, as an ideal, etc. What are your acceptable risks, what is not acceptable. do you need to comply to any specific compliance requirements (or is the near future need to) such as HIPPA, SOX, ISO, etc.
Ask what kind of budget you have to work with - this is SUPER important, without knowing what you have to work with there is no way to set priorities or plan for tools and equipment.
You now have enough information to start planning and building your new environment.