You won’t need a separate range extender. For both zigbee and zwave, pretty much any mains powered devices except smoke sensors will automatically act as repeaters anyway. The most popular are usually light switches or plug-in pocket sockets, but also in wall micros, plug-in sensors, plug-in sirens, etc.
Battery operated devices do not usually act as repeaters.
Z wave repeats only for Z wave and zigbee repeats only for zigbee, so just take that into account when deciding which devices go where.
For more information, see post 11 in the following thread (the topic title is a clickable link), then go back up to the top of that thread and read the whole thing.
(www.rboyapps.com - Making SmartThings Easy!)
For a Z-Wave thermostat, the GoControl/Linear/RTC CT-xxx series thermostats are great, cheap, highly configurable and easy to integrate. The ZTS-500 series thermostats are also good but more expensive. You can also look at Nest, EcoBee and other utility specific integrations.
For Sirens, I would recommend the (Lowes Utilitech also sold as GoControl and some other brand names I believe) Siren, it’s one the best and has great battery life. You can also look at the Vision Siren that you can buy at www.thesmartesthouse.com
Range Extenders, JD has given you a great guide to check out.