The gen 1 250 and 300 HP motors are no longer in production, everything from most 150's through the 300 are G2 models today. If you are purchasing a larger G1, then it would be from a dealer's remaining stock.
It doesn't matter which generation it is as either is eligible for the 10 years of factory coverage until March 31.
Make sure the motor is propped for the correct rpm and type of boating that you do. If possible, have the dealer go with you for the initial boating test run to make sure that everything is set up correctly.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.