The 850/960 etc. engines are indeed interference designs and one does not want to suffer the consequences of a broken timing belt.

Following Volvo's recommended time/mileage change interval should be sufficent.

I changed mine myself in about 2 hours and the belt cost was around $50, so for me it is not a big deal. It is not a hard job for an experienced DIY mechanic such as myself, but I would not recommend it as a DIY project for the beginner, mostly because getting something wrong can have bad consequences.

The Volvo 4 cylinder motor also requires periodic timing belt changes. My recollection from when I last did the job of our old 240 was that the job was similar in difficulty to the 850, except that on the 240 one needs to remove a big stubborn bolt on the crank pulley. My air wrench made easy work of that, but without an air wrench it might be a problem. The 850 is front wheel drive, and getting at the belt is a little more of a pain than on the 240. However, the 850 has the modern style single belt for all accessories which is much easier to remove for getting at the timing belt than is the rats nest of traditional V-belts on a 240/740/940. Pluses and minuses in both cases.

John