Need an advice with mesh wifi for my company small office

Hi everyone,

The story is my boss wanted to replace all the wifi system in the office with the newer ones. They're using the old Linksys WTR54G for this purpose for a long time (I just worked here recently), and they're slow and becoming unstable now, plus just a small office but we have like 5 different SSID, it's annoying when you have to go up stair and then down stair then had to manually reconnect the wifi. So I recommended my boss using the mesh wifi system and he agreed. The problem is I do know about this system but never really had a chance to build a network system with it, and I kind of lost between a lot of manufacturers and brand Unifi AP, Linksys Velop, Netgear Obi,... so I need you guys advice.

My company office has two plans (two floor?), about 405 square meters (27m x15 m) (about 4300 square feet). 20 cm brick wall, 3.5m (11.5ft) from the floor to the ceiling. And the blueprint of the plans are below. I'm thinking about using 4 Linksys Velop, 2 in the ground and two in the 1st floor. Is it enough to cover all the office?

ground floor blueprint: https://imgur.com/nX3MdSh

1st floor floor blueprint: https://imgur.com/EjtxWyj

I'm appreciate any advice, thanks.

p/s: english is not my native language so sorry for any mistake :D