Hope you enjoy your stay here on the msuk forum it is packed full of infomation, on new cars, new setups & new gear!!
The Tamiya chassis is basic so running a 12turn motor may shread the gears as they are only cheap plastic is used for them. A 19turn motor is probably the best bet as they are efficent but also fast!!
When it comes to pinions (motor gear), there is only a limited ones you can use, normally on the M03 chassis it will say 19, 21 & 23 these numbers relate to the number of teeth you can use on a pinion.
So in your case 19tooth, 21tooth & 23tooth. These numbers will be dotted around the motor mount were there is 6 holes. one pair is for each pinion to insure you get the correct mesh for that pinion.
Esc from mtroniks are great for beginners as there small, and get you going for not alot of money.
Hope this Helps
Best Regards
Hyper-Harry