Try to keep it in perspective, the people who do have trouble are vocal about it. There are literally hundreds of thousand of owners who don't complain on internet boards. The car has earned it's reputation and with care you'll most likely have a good experience.
In many areas there are garages who specialize in Lexus and Toyota and can give excellent service at far less than dealer prices. Do some research *before* you need anything and ask around for your local guru. :)
Like you, I read the horror stories about 5000 dollar transmissions jobs and maybe people pay those prices but it is just a Toyota tranny, millions of them out there, how much could it cost?
I checked with a local guy who has done transmission work for me in the past and he said he could do a complete overhaul for 1250, 400 labor, 600 parts kit, 250 for a TC. The point is, plan ahead, make contacts before you need them, don't be at the mercy of the dealer.
Have the dealer run the service records for you, see what has been done to the car. If it's made it 100K its probably a pretty good car and seeing maintenance records can let you know what may be coming up. The RX is pretty predictable as far as what consumable fails when.
As far as the price, take heart that a dealer sold it. Lexus dealers have a choice to make on their used cars, questionable cars are wholesaled, the good ones go on the lot, if they thought you were going to be haunting them with that car, it would have been shined on down the road before you ever saw it.
There's a lot of negativity on the boards from people who have had bad experiences but try to balance that with the thousands who haven't found it necessary to come here and complain. :)