I tried to find a used engine in UK with no success, but I would think more 4.6's were sold in the US, probably something to do with your gas prices . However if I could have found one thats the route I would have taken. If the engine you have found is warranted with that mileage, and you will have a warranty after its fitted then thats the best way in my opinion. After a complete strip and repair of my 4.6 I think I was into it for around $6000 in parts alone. Luckily I could do the work myself, but when you take an engine apart you will always find stuff you may as well change while you are in there.
You could always have the engine stripped that you remove and see how much it would cost to put right, and then sell it