Have a knowledgeable technician have a look at the van for you.
P0702 is an SAE OBDII code read from the engine module, not a native Mercedes code read from the transmission module. All it tells you is *something* is wrong in the transmission control module, but not what. The 45mph speed cap indicates the same thing - the transmission is in limp-home mode and self-limiting to 2nd/Rev gears.
A Sprinter-specific scan tool can pull the native Mercedes fault codes directly from the transmission module, giving you a better picture of the extent of the problem. Without that information you’re running blind and can end up throwing expensive parts at the van.
That said, if the engine is running and idling smoothly, this may be limited to the transmission. Common issues are dirty magnetic sensors on the conductor plate, and fluid leaks causing electrical shorts at the module. Both are relatively easy to address with a new conductor plate, connector plug, and fluid change. This is a moderate DIY level project since you need to drop the pan and valve body, but is within reach if you have a clean working space for about US$250 in parts and supplies. A shop should be able to get it done in about two hours? There are specific fault codes that will point you in this direction.
Good luck with your evaluation of the van,
-dave