Just starting looking for direction, I’ve been install building automation systems for close to 30 years , schools,universities and industrial. Andover system. So far I have a few wemo devices and just ordered a few sonoff devices. I have a raspberry pi 3 from a previous project. Thinking of getting a z-wave devices and a thermostat. . I’ll need a z-wave hub.
OK, sounds like you might be in the UK? If so becareful about buying in-wall switches as the majority of UK houses don’t have a neutral wire to the switch as we use switched live (so there is only live going in/out of the switch), new houses or rewired houses may have the neutral wire depends on when they were built/rewired).
If you go the zwave route make sure you have some wired devices (only wired devices allow for meshed networks to extend the network range, battery sensors for example can’t be used as range extenders). My take on zwave is it’s very expensive atm, it may come down over time tho’. The major problems people seem to have is configuration, once setup the network works fairly well. Battery sensors can be a problem as they will loose battery life and don’t always tell you they are running low 
Theromostats I don’t use so can’t advise on them, I’m sure someone else will be happy to point you in the right direction 
As for z-wave hubs/sticks there are a few out there - I have a generic brand stick that works well, others are available :O) Make sure you buy one that works where you are - the freq’s differ for different parts of the world.
RPI3 is good for most things, it’s probably what most people use round here , myself included, tho’ they can get a bit slow when heavily used, but for £35 you can’t go wrong there really and since you have a spare, you might as well use it
Sonoffs are great I have few I program them via ftdi but now you can do it over the air (OTA), and to get HA to switch them you need to reprogram them.
It’s a bit of a learning curve but great fun
On a slightly different tack you might want to consider xiaomi devices/sensor (and a hub) as they are very cheap compared to zwave). In fact it maybe worth going that route alone for the door/window and occupancy (PIR) devices obviously depending on how many doors/windows and rooms you have
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I’m in the USA and electrician by trade. Wiring is not a problem for me but the software I have to work at it.