You might want to look for a company who runs automatic lawnmowers. You can get a good first hand exercise from them.
I am surprised that your client hasn’t asked about lawnmowers. In my small town there are at least 3 gardeners who takes care of private property gardens all year long. One of them runs a business, where he sells plants, trees, etc. He used to have an automatic lawnmower too for display, until someone stole it from the front grass. (By the way the whole thing was recorded on camera by the petrol station next to his business.) Anyhow he was advising people if they wanted to buy automatic lawnmowers.
Smart irrigation is a cool thing. You can get many brands doing the same thing. Weather forecast correction and soil sensors makes it really smart. Orbit, Rachio, Rainbird, Spurce, Gardena the brands to look for. Some of them supports direct change from previous controllers. Gardena has a whole smart garden line as I know.
For lighting, you get better to talk with architects and electricians. There are plenty outside lighting solutions around. Some works as smart itself. Some can be made smart by wiring the switches to relays. Existing lighting solutions can be upgraded to smart setups too.
For weather stations you can get “cheap” ones, like FineOffset clones. Like Froggit WH3000. Or you can go for expensive Davis weather stations. Both can report to Weather Underground or to the Met Office. But I have a doubt you really need a weather station for a smart garden setup as tou would use internet services for weather forecast. But for the tenants, it gives a nice thing when they can look at their own place’s weather.
But really the question, what your clients want as “smart”. Fixed automations? Do they want to save electricity, water or manpower?