that would be true of things like gm engines where the water pump runs off the cambelt, but that's not the case on escos. if it seizes, auxillary belt will slip, but no damage to valves etc (unless driven while water pump is faulty)

i've never had one seize. usually leak around the shaft seal first in my experience. if you know that it's been well looked after and had regular antifreeze changes, then i would leave it. if history unkown, impellor could be all shitted up and may be worth changing

once you have a price - and i have no idea how much or where from but would probably start at woodford - then you can make a decision