I have my doubts that you'll find a metal aftermarket radiator for an 850 (Nissens?) but overall, the OEM plastic tank/aluminum radiators are really very good and I'd just use another new OEM radiator myself. Although not common in Volvos, leaking trans coolers are not a totally unusual thing to have happen in any radiator sooner or later.

Don't drive the car anymore as is.

When you get the new radiator, if you do it yourself, get some cooling system flush solution (like Prestone) to flush the ATF from inside your cooling system but before you do that (run the engine), make sure there's no coolant in your trans (drain and fill, more than once or flush until it's red again).