I think for $475, I'd do some investigating on my own to determine just what is the problem. It could be something as simple as a seat safety switch, or not sitting in the seat properly, or another safety switch like trying to start with the PTO engaged, or what have you.
Could be you broke a wire somewhere along the interlock circuit in using the tractor.
100 hrs on a starting solenoid is wayyyyy to early to suspect a defective unit. If it was defective in mfgr, it would have shown up much earlier. I had a 2400 and never had a starting issue in the 4 years I owned it.
The starting relay (solenoid) has a 12v energizing low current wire which , with power applied with a ground applied causes the solenoid to energize. You can disconnect the energizing wires at the solenoid terminals and, with terminals or other "test" type accessories, connect power to those terminals and determine if the solenoid will close and roll the starter. If you do and it does, then one of your interlocks is open. If not then it could be a problem with the high current power connections to the starter, which includes the grounding circuit from the starter case back to battery -, or the battery itself.
If you don't know what I am talking about, do you have a neighbor with some electromechanical abilities to help you?