6.5, 5.5, 4.5 etc means nothing as every motor is different. Brushless is not like brushed and turns don't mean anything really. For some reason the rc car industry is still using them.
Anyway with brushless you can basically gear for top speed anyway, so depending on what system you go for, 4.5, 5.5 or 6.5 will be fine. Acceleration is not greatly affected like it is with brushed motors. As for LiPo, you are right, bigger packs can supply more current. I use a 4900mAh Trakpower in my cat, as this is BRCA legal. The 5400mAh are not BRCA legal, so you won't be able to run at places that follow BRCA rules. It's good to have some weight in, as LiPo is lighter than traditional NiMH/NiCad stuff.
I would recommend the Tekin stuff, it's awesome, but Losi is fine. My brother races with a 6.5 losi and it's good for 1/10th off-road so you will be fine with it.
