Start small, add gradually. Pick one ecosystem and stick with it.
Ecosystem
Alexa: widest compatibility, cheapest. Google Home: great voice recognition. HomeKit: strongest privacy.
Smart Plugs ($15-20)
Any outlet device becomes smart. Lamp on sunset schedule. Coffee maker with alarm.
Smart Bulbs ($20-40)
Brightness and temperature control. Put where you change lighting most.
Speaker ($30-50)
Voice hub. Echo Dot or Nest Mini. Display version adds camera feeds, recipes.
Thermostat (~$100)
Saves money. Learns schedule. Pays for itself in energy savings.
Cameras ($30-60)
Wyze, Blink, TP-Link. Motion alerts. Door sensors.
Automations
Goodnight: lock, lights off, thermostat down. Leaving: everything off, cameras on.
Total: ~$200-300
Add over time for actual needs.