If you're willing to go all-in, get a big technic set which has the most "power functions" components (motors, remotes, battery box). These are the parts that are harder to come-by unless you order them individually (which is more expensive).
When I looked >= 1-2 years ago, the best single set for spare parts I found was the Volvo L350F wheel loader aka. "42030" [1] (each lego set is uniquely identified by a numeric id). It had the most power functions parts I could find, and probably gives you all the beams, wheels, gears, and other parts you need.
You can supplement your collection with specific parts from places like BrickLink [2] or even the lego.com store [3].
NOTE: Be warned that LEGO is changing their Power Functions plug shape. Most current sets are still on Power Functions 1.0, and new sets will eventually become 2.0
[1] https://www.amazon.com/technic-Volvo-L350F-wheel-loader/dp/B...