1. The Amazon price is probably actuator only, rather then the whole valve. Not sure you can get them from Amazon at the moment.
2. If only the actuator is to be replaced, its a relatively simple job assuming the wiring centre is in a reasonable state. Say 45 minutes work at worst. £159 is not cheap, but its not outrageous.
3. If the whole valve needs to be replaced, the system would probably have to be drained, then the old valve disconnected and removed, the new one installed and connected and the system refilled. If all went well, perhaps an hour, with complications could be three hours, particularly if there is difficulty draining / refilling the system. On balance a cheap to very reasonable quotation.