I'm not sure what your concern is here. If you are worried about the delays caused by the infrastructure between the 2 ends, then you will probably have problems with whatever arrangement you have in place to connect the 2 end points. 200-300 metres is not that far really (especially when you can get connectivity around the world!)
If you are worried about the number of items needed between the 2 end points, then this really has nothing to do with the information being serial or not. Ethernet connections do have a limited (distance) span but it would help if you could tap into existing infrastructure for this.
Of course, this assumes that you have existing infrastructure in place but your requirement to use Ethernet implies that this choice has already been made. If not, then could there be some other medium, and some other protocol, that could be used instead?
Susan