It's really disappointing to hear that the transmission went out before the clutch. If that is truly the case, I have just become less impressed with this little car.
Do you trust this mechanic?
Sounds like you are receiving a very vague answer to your problem. How do they know it's an internal transmission problem?
Maybe they have explained that to you? I would consider a second opinion if my mechanic is not a transmission guy.
Sounds like you are done with your Mirage, regardless. I would love to see a forum member purchase your car & keep it running. Don't you have a clutch kit to go with the car, too?
I hope someone on the forum buys it, but it won't be me. Hope you enjoy your new car Steve!