Gearbox problems.... replace or repair ?

I have a 2000 Jaguar S-Type with the 5R55N transmission. The engine is a 3.0 V6. I bought the car two years ago and after a while a strange sound start to be heard from the transmission, like a wheel that touches something metallic. The sound could be heard as long as the engine was running, and was stronger when I was pressing the acceleration, no matter the gearshift position or if the car was staying or moving. In the same time I start to feel a shock when the gearbox changed into the reverse. The car has 150.000 km now.
I decided to change the transmission fluid and the filter, so I bought the filter and 6 liters of Mercon V oil and went to an automatic gearbox specialized service. When I was near the service, the metallic sound stopped, so the mechanic that performed the oil change cannot hear the sound. Anyway, I left the service happy that the sound has gone. The shock still remained, and sometimes a delay occurs since the gearshift was placed in reverse until the gearbox answered. At the beginning of the summer, after a longer trip, when I arrived at home later in the night, the reverse was no longer responding... I accelerated in N then changed into R and the car started to move but I could feel like the gearbox hesitated and one second was in reverse next second not, then again was and so on. It was late so I just parked the car and leave it. After two days, I tried to start the car but it was impossible. I started the engine, but there was no response in any position : R, D, 4, 3, 2. I presumed it's something with the shifter cable but it was OK... The following days I tried a lot of things but there was no response. So I leave the car there until few weeks ago when I started the engine and I was just playing with the gearshift (having the brake pedal pressed) when I felt the old reverse shock.... so I released the brake and the car started to move. I selected "Drive" and it also worked.... The next day I take it out for a short trip and it worked fine until I kicked it down, when "transmission error" message appeared. Now the car "works" but the delay before switching into reverse is very long and it seems to me that the metallic sound appeared again. I asked few specialists and they told me that the repair will cost me at least 1000 Euros, but could be 1500 or even 2000, depending what they will found inside. I also found some second hand 5R55N gearboxes, the cheapest one from a 4.0 S-Type.

What's your suggestion ( considering that I cannot be sure that after few kilometers the "new" transmission will not fail) ?

Thanks !
Emanuel.