Do I need a new router to go with ubiquity APs?

-- edit -- autocorrect in the title which I can't change, Ubiquiti

I work in a small office - 8 people. We’re running a BT router (I know I know) but need to setup a guest network. Honestly the router is fine. We have FTTP, and never really have any issues - other than a back room which can’t be reached - and is currently using a shonky netgear extender. We need pretty good speed, we regularly are transferring large files, or large volumes of files, or working with online software.

The main working area needs good WiFi to the main network (although most people and the server connect through a network switch). The back area needs the main network and the guest network.

I’m thinking about getting Ubiquiti gear. I know computers, but am a complete networking novice, so excuse the stupid questions. We have an IT guy who is will set it all up for us, but he's a little oldschool and I don;t really trust his choice of gear so much. It's a remote location so we don;t have a lot of choice in terms of IT people. Understandably he wants to recommend things he's used, but the last recommendation he gave us was for a draytek router that packed up after a couple of years. I Just want to make sure I buy the right things... So my questions are:

• In order to get a guest network do I need multiple access points? So in order to get the back room good signal on both the normal and the guest network would it need 2 additional aps? Meaning 3 in total, one for the main area and two for the back room.
• which APS do I want?
• do I need to buy a new router too? the edgerouter(lite)?

Ubiquiti's site is pretty confusing. Im confident with the right gear in front of me I could set it up tbh. I'm happy to spend what we need to get good gear - but I don't want to overspend buying something that is for a football stadium, or an office of 100 people, when I don't need it.

Thanks