You could search this forum to learn of some of the woes people have experienced with middle aged RXs but these were all one offs so far as I can recall.
You really are down t the usual second hand car checks which I recall include service history tyres, MOT and obvious signs of neglect, accident damage repairs and so on.
You seem to be aware of the obvious downside, high fuel consumption (use BP Petrol not supermarket urine to improve it to c20mph) and it may spring you a nasty surprise as any car of this age can.