If you want to guarantee E+, buy a subscription.
Don't expect upgrade (to the front of the plane, be it domestic first class or business class). Even with miles + copy, priority waitlist ranking for Silver is not in your favor. Does it happen? Sure...even sometimes instant with positive upgrade space, but it is not the norm.
Also, adhere to your company expense policy. My company prohibits paid upgrades and seat subscriptions with company funds. Your company policy may vary.