Are you using the US or the U.K. hub? For the US, it’s pretty easy to do the whole house in Zwave if you want to. There are certainly lots of zwave light switches and outlets to choose from.
There is a hard limit of 232 zwave devices per network, which is one reason that, although there are some Z wave lightbulbs, most of the lightbulb manufacturers are more interested in zigbee, because then they can sell it as a solution for office spaces and commercial buildings as well.
All of that said, if you are going to use a smart switch, you generally will not use a smart bulb. You will just use a regular dumb bulb. The only reason to go with the more expensive smart bulbs are:
you want the color changing capabilities of the smart bulbs
you want to set up zones in a large room like a basement where all the lights may be on the same switch
you are renting and are not allowed to replace any wired devices
you just don’t feel comfortable replacing switches and would like a non-wired solution
Otherwise, a smart Z wave switch plus regular dumb bulbs will probably be the least expensive choice.
Many community members, myself included, have some zigbee smart bulbs and some smart switches of other protocols (Lutron are also popular, which is what I use in my own house) , and each is just selected on the needs for that particular location.
Have you had a chance to look at the device class features FAQ? It might also help answer some of your basic questions:
I personally don’t replace every switch just for budget reasons. I typically replace one per room. But there are other people who do replace every switch. Different things will work for different households.