Adam welcome to LOC!
I am like you have had an e60 525d, 2009 and had the same thoughts like you - the car is getting to that age when it will be asking for lots of money! Saying that the e60 I had was one of the most reliable cars I had (3.5years)! Loved the handling and that 6-cylinders engine was very good!
My 1st impressions on Lexus were that it is much much comfier, steering wheel is much lighter, no diesel vibrations and in general the car was quitter and that Mark Levinson stereo is so much much better than standard BMW audio, I love it! On top of that my Lexus has double xenon lights, rear blinds and cooled seats!
I still love the shape of e60 but Lexus GS so much grown on me as well, I do find Lexus a very good looking car!
So try it, I am pretty sure you won't regret it!
Keep us updated Adam, good luck!