I am on my 5th "older" Subaru in about 22 yrs... and the FIRST thing that gets done with them when I purchase is a full timing job.. regardless of history.
you may have proof the belt was done, but, were the pulleys replaced? how about the water pump? seals? those are often not touched, and on the 2.5s the cogged idler pulley is usually the one to go first from bearing failure. when that happens, you end up with major damage costing a couple thousand to fix.
the only way to know it is done, and done right, is to do it yourself - either you personally, or a good shop you trust. dont rely on anything else.
my list spans from a 1989 GL wagon (that i paid $150 for and drove for 8 yrs, thanks to this forum) to my current, a 2004 Forester... all were purchased used, and mostly with no real maintenance history. All had somewhere between 155k and 214k on the odometer. A bit of preventive maintenance to start out with prevents a whole lot of headaches down the line.