most cars will last a very long time if the maintenance schedule is followed. However some cars are beter than others this vw is suspect. The best way to put your mind at rest is to look around, how many do you still see on the road or for sell. If there are not many are cruising around you have your answer.
Some Vehicles like the Cabriolet are seen near the coast line where the weather is best for driving with the top down. Don't get discouraged if there are few in your area.
VeeDub's have always been built well and last a long time. The engines can easily do 200,000 mile if they have been maintained well. Always open the bonnet and look if it has been cleaned as they could be trying to hide something. Touch the top of the engine and check that its cold so you can start it and see if blue/white smoke comes out the back. If its hot means they might be trying to hide something again. If the cabrio has a clipper kit always try and have a look around the joins were the kit fits the body as it can hold water and rust. Check the tops of the wings because if it doesnt have a underwing shield the water just gets up there and rust. Always check the lines of the car ie. by looking down the sides by standing at the front or back. Check to see if all the panels flow into one and other. Try by looking down the mording chich gives you a good line along the car. Check to see if the door, bonnet and boot shuts are all about finger thickness apart. Check the crumple zones at the front in the engine bay. Look for welding or if they look as thought they have been crushed and pulled. check the boot to see of any signes of been changed or pulled.
I hope this is helpfull in some way.
Enjoy the sunshine with the roof down as i do When ever it comes out ;0)