Okay, heres my opinion (If needed!)
The Puple Heat motors look ok, but are a bit small, no ive never used them, but i got the size from a near-by ruler!
Can we have a link or picture of these Gold Motors?
Drill motors might be a good choice, go down to your local Wilkinson, and they do some quite cheap drills there, get 2, then, take it all apart, get an adult to help with getting the end bit off of the motor (It should come off just by pulling or twisting, but its so hard, im 12 aswell, and i did Ju-Jitsu for 4 years so im pretty strong, but i still cant do it!)
They would be a good choice of motor, fast and reliable (Sort of).
We used these motors in Hunter design 1, but then it broke itself, so now we use them in Chicken Supreame.
Its really your choice.
Hope this helps
Matt
Team Hunt