I'm pretty sure that 99 is a split year regarding the type of tensioner. Early in the model year had the hydraulic tensioner and then Volvo switched to the mechanical type. I had a 99 V70 and replaced the timing belt, mechanical tensioner and the idler pulley. The water pump was in good shape so I left it alone. It was a pretty easy job but access is kind of tight. The crankshaft timing mark is difficult to see and I was a bit worried about making a mistake putting the new belt on but no problems. My car had about 100K miles when I changed the belt and that was in 2007ish. As I recall, the original belt was in pretty good condition but I was glad for the peace of mind that a new belt provided.

I'm not going to recommend if you should change your belt or not, that's something for you to decide. Milage wise, you are in limits. Time wise......
Also, keep in mind that if the T-belt breaks, you will have engine damage. The valves and the pistons will attempt to fill the same space at the same time and something is going to give.