As to the e-bay boiler my crystal ball is all steamed up at the moment. It will be pot luck as to whether it holds water or not just like the boiler you already have.
I can see the two long stays don't have the bushes soldered in place. You will also need to drill & tap for cylinder and weigh shaft bracket, door hinges, pump, etc. It may be a slightly different size to yours as boilers move during construction so your hornplates may need re drilling and if the width is different you will have to play with shaft lengths. You will have to drill out all the rivits on your smokebox and drill and rivit it to the new boiler.