Just as a point of interest regarding gaskets and seals drying out. The days of rawhide, graphite impregnated rope and cork are gone. Todays seals and gaskets rarely leak unless something damages them or the installation was poor initially.
As far as age related deteriation my concern would be for exposed items like fanbelts, radiator and heater hoses, air lines, etc. Universal joints and automatic brake slack adjusters along with ball joints on IFS coaches seem to either get neglected or over serviced in the case of slack adjusters.
As Bruce said in the previous post I would be a little slow to pull the trigger on a coach with the ISB Cummins. Lower torque and a shorter lifespan for the small Cummins are concerns for many. In heavier coaches a life expectancy of 150,000 miles seems realictic, where the ISC and other bigger Cumming have a long history of 400,000 miles before overhaul, and many documented cases of a million miles plus.
The real story is simply you can have good experiences with a young high mileage coach and the same with a old low mileage coach. There is no right or wrong answer. Find what you like and jump in and enjoy the experience. Don't let fear drive your fun out of your life.