I think hes talking 12k for the total bike, frame swing arm, etc. not just the motor.
You have a long road to travel and it may take you several tries to get it right. You may go a direction and it may not be what you want, so you start over buying new pipes, carbs, cylinders, crank, etc , basically starting over. your best bet would be to setup a nice chassis that is going to handle whatever you would want to throw at it in the end. This way you have a good starting point. There is nothing scarier that a fast motor in a POS chassis. There are certian motor upgrades like the override and lockup that will work with almost any engine combo, but then you have to chose do you want a 1-5 or a 1-4 manual 5-6 or whatever. hope that helps and good luck