Just to chip in, I would agree with your plan to get the G4 laptop.
it's a great computer and you'll still get many years of good use out of it - check it out carefully, you may want to invest in a new hard drive and/or a good backup solution to ensure optimal usage going forward.
I would look to put the cash you now have into projects where you know a lot about the area. IMHO you don't know the values of various computer systems, so trading in this area does not look promising to achieve your objective.
Quite useful would be to work backwards: you now have a great laptop for high school, you just need to generate the cash for the car renovations + insurance + fuel + running costs. Take that total, and work backwards to what you have now.
For example if it will cost you $5,000 to renovate +++ and you now have $500, and you decide to make cash by trading in an area where you can get a 25% profit each time you flip a deal, then you need to make 11 deals to get you to $5,821 as illustrated below:
500
625 1
781 2
977 3
1,221 4
1,526 5
1,907 6
2,384 7
2,980 8
3,725 9
4,657 10
5,821 11
However, I would suggest you do not risk all your capital in every deal. Lay away 50% of your profits from each deal into the 'Car Fund' which you never touch. So, in the illustration above, you invest $500, make 25% return which is $125. Out of this, you put $63 into the Car Fund, and now have $563 to play with.
The balance you invest into your next trade. Don't worry if sometimes you get a 15% return, sometimes you will get a 35% return to make up for it.
This way, after 21 deals, you will have put $5,432 into your Car Fund, and still have $5,932 in trading capital.
I would advise you read
Rich Dad, Poor Dad for Teens by Robert Kiyosaki. A very useful investment!
Hope this helps, and good luck, entrepreneur!
