The thermostat has a gasket, so if you take it off you'll need a new gasket. It shouldn't need any sealing but I always use some Blue Hylomar as an extra precaution and I never had any secondary leaks.
You'll probably be able to fit the new hose in situ' though as a new one will be all lovely and soft and rubbery. Your old one looks a bit aged and brittle, that's all.
You just need to clean up the stubs on the thermostat and the rear block and slip it on. Use new Jubilee clips but put completely unscrew them (i.e. open them up) and put them on afterwards, rather than hooking them on the ends of the hose.