There is an easy, cheap solution to using lipos with older escs. You can buy cutout devices (Novak and LRP do them) but what the flyers do is fit a simple warning light.
The whole thing cost me £6 and its basically a voltage detector and a bright red LED. You get them in set voltages for 1S, 2S, 3S etc. The 2S goes off when the pack drops below about 7 volts (3.5V per cell)
You place the LED somewhere visible (I used the window) and stop when the light comes solid on - it will flash sometimes if you accelerate hard as the voltage drops due to current draw.
As for what pack to use - C rating is much more important than capacity. Because Lipos have lower internal resistance than NiMH you actually use less capacity ie a 4200 Lipo runs longer than a 4200NiMH.
Couple that with a brushless motor and I've managed to complete a 4wd race, outdoors with a 5.5 motor powered by an 1800mAh pack from a micro! So pretty much any size will do.
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