JoJo28

I have had my 2001 RX300 (auto) for almost 5 years with no issues. As soon as i started looking at trading it in for something newer, it broke down!! Was driving absolutely fine and all of a sudden began revving up to 6-7000 revs! Once the ignition was turned off and restarted, it run fine for a couple of miles, before revving up again and losing power. The AA came out and advised that my gearbox oil was sitting much higher than the limit on the dip stick and i have never topped this up! The AA technician advised it must be clogged and that the filters etc may need cleaning. I was towed to my local garage and advised that as the gearbox oil was thick and black, so the oil was contaminated and therefore a box rebuild would be required costing £1700 plus £600 labour and £100 sundries plus VAT. Absolutely gutted - just been offered £900 from scrap company! Is it worth repairing????? Any thoughts or suggestions? Many Thanks,