Disconnect water pump, apply 12v, does it spin?
If it is knackered, and you need to replace it, then ask yourself this: What does the part do, and what happens if it fails? In this case it could mean your engine overheats and goes pop, so that should answer your question as to whether you should replace with OEM or a £50 eBay special
If your coolant temps are fine, then I wouldn't worry about it. Engines get hot under the bonnet, unless you have other symptoms I'd not give it a second thought.