Yes, We're Time Travelers - N. E. Ottinger - Bog

A former Navy SEAL instructor and NAU physics professor, Dr. Henry Spere, hacked a Munich research lab's file right before its destruction. Months later, he wins a court challenge to his inheritance money from an unknown heir. That event is his call to develop the technology in those secret files.
The Rinklers introduce him to Rex Tavendor, who becomes the Professor's assistant.

The Rinklers do the upkeep on the farm property while the inventors work on their storage container time machine. The Professor's military mindset makes him astute at staying under the radar of bureaucrats and corporate oligarchies when purchasing electronic hardware and supplies. The team must hurry to "get out of Dodge" before elitists haul off their technology in a dreaded Waco-style raid.