Genuine Volvo is a good idea for sourcing the T-stat, but if you can find a Wahler unit that is fine as well. In fact, if you get one from Volvo, inside the blue Volvo box you will find ... a Wahler thermostat. So, really, you don't have much of a choice. All roads lead to Rome....
Also, be ready for a pretty big mess when you take the housing off, and again when you pry the old T-stat out of the block. Since the VW/Audi (/Subaru) convention puts the thermostat low down in the cooling system, you lose a lot of coolant when you replace it. Still an easy job but not quite as painless as a redblock or whiteblock Volvo with the t-stat right up on top, where you only lose a pint or two of fluid when you open it up. Overall the design difference is worth the slight added mess and hassle though, since it means if we run low on coolant in our diesel Volvos we won't have the thermostat sitting high and dry and consequently closing just when coolant flow is needed most... like our friends in the gasser Volvo world might experience.
Also worth noting -- Most D24T's from 1984 onward came with a nice little coolant drain petcock located in the plastic tee piece where the lower cold start thermostat hose connects to the heater return hose, just adjacent to the thermostat housing. If your car has this, it may help keep the coolant mess contained somewhat if you drain the engine using the petcock before taking loose the t-stat housing. The petcock is even set up so that you could stick a piece of hose onto it, to direct flow straight into a container of your choosing. Just another one of the nice little thoughtful Volvo touches that you find on these cars from time to time.
Be careful to counterhold the female side of the petcock assembly with a second wrench, though, if you decide to use it. The plastic tee piece that the drain is located in can break if it is asked to absorb the force of loosening stuck threads...