Odd, i have games in the year 2500+ that I can still buy vehicles for.
Sounds like a problem with the save or the mods are causing a conflict. Try a new map in 1950, time warp to 2051, then try building something again.
If you want a 2050+ year save you test with, you can use this one: https://drive.google.com/file/d/0B9uFvN9K3tPWMktUMERrN1dLUDg/view?usp=sharing