3) for me the most "all-round" motor would be the 2.6
2) i would choose the transmissions, but only if you buy a more potent motor, because with an 1.0 motor the diference between them is not going to be very big.
1) if you want to lower a little bit you shoud check this: http://tinyrc.com/qfm/racing/mod_01.htm#03