I have collected several ECU computers from 850 T-5R, 850 R, X70 T5 manual, etc. These are believed to be working, most were removed from running vehicles, a couple from the salvage yard. These ECU's rarely go bad.
If you have an 850 turbo and want to add power, you can purchase a R ECU and add performance, a R or T5 ECU will give you more performance over a GLT ('97) programmed ECU.
NOTE: A '96 850 ECU should work well in a '97 vehicle, but a '97 ECU will likely give a code if put in a '96 vehicle.
Interesting fact, the 1998 T5 and 1998 R ECU's are programmed at the same boost level and may carry the same performance. So, I don't think there is a reason to get a R ECU over a T5 ECU when dealing with a 1998 x70 vehicle. The GLT and XC vehicles will benefit from a T5 or R ECU.
If you have any questions, feel free to call or text me. I also have a video on YouTube about the ECU's. "Search R ECU replacement for Volvo's Robert DIY"
WARRANTY: If any of these ECU causes a CEL code it is likely due to a different emissions system arrangement in your vehicle from the vehicle the ECU came out of. If that happens, you can return it for a purchase price refund or exchange it with me if I have another that should fit your need.