[Finnster]

Personally, I like to work backwards from my expected wattage, and design from there.
Knowing how much wattage you need is the tricky part, but with some experience or reading Eagletree threads for similar trucks, you can get a ballpark.

For a 1/8th scale MT or truggy of average weight, to get 40ish mph, you will need 1600-1800W peak. To get to 45-50, you need 1800-2100W. The "at speed" wattage will always be low, and you will spend most of your driving accelerating, ie mostly peak wattage, so I don't worry about that.

So, for 35-40 mph in your example, lets figure 1700W.
Now just decide what voltage you would like to run at.
4s? I figure good lipos will hold 3.5V/cell at peak. So, since W=V*A => A=W/V:

A=1700W/(4*3.5V)=1700W/14V=121A peak.

Always overdesign, so given CC's advice of 80%, simply devide 121A/80%=150A (OTOH 121/.8) Just like Linc said.

If you want to be weird and run 7s? 1700W/(7*3.5V)=70A 12s? 1700/(12*3.5)=40A

Ok... So what motor? I like to run my gearing around 13:1 or so, and motors around 28K-32K. Some people like to wind the hell out of the motor, which is fine for big speeds, but doesn't seem to make eff setups for lower speed apps, as you have to lower the gearing alot. 20:1 in a truggy is kinda dumb IMO. Gearing should be independent of kv and voltage specs.

So, figure 30000rpm / (#cells*3.5V) = ballpark Kv.
4S: 30000rpm/(4*3.5V)= 2150kv.

You can do this the opposite way to guess gearing from desired rpm, but you should have some idea of where good gearing ranges should be. 11:1 to 17:1 is a normal range for trucks.

So you got cell count, and desired amps, and the Kv picked out. For batts, use the continous rating versus what you figured your peak amps to be. You will mostly be either peak or off throttle when driving, so only worry about this.

Batts:
For runtime.. You can use needed C ratings as a relative measure to guess runtime. If you design your setup to require 40C batts, runtime will be short. At 30/25C you will get an avg runtime and 15/20C will give you long runtimes. For racing you probably want to size the lipo figuring 30C values or so (depending on race length.) For bashing I would figure ~20C or so where you don't worry about overly heavy batts.
Ie. need 150A batts. If bashing, use 20C, 150/20=7.5 7.5Ah batts will give long bash sessions. If racing, 150/30=5. 5Ah batts should make a 10min race.

For 4S we said 150A was our expected draw. Ensure the lipos you choose have a continous rating close to this. For 5Ah batts, you need 30C min. For 8Ah, you need less than 20C. There is some play in this. Its not like 5Ah 25C batts won't work, but 20C cont/30C burst will be pushing it. If they are batts like Neus and Hyperions and whatnot that have much higher burst ratings, ie 25C/50C, a continous draw will be fine. If they are chitty batteries like MA, better go up in C or Ah ratings for safety. Actually, if they are MA batts, they lie about C ratings so much, just go up in Ah, then kick yourself in the nuts for buying such expensive, but chitty batts. ;)

At the end, this gets you close enough where the final tweeking can be done with gearing to dial the setup in. You can always cheat the numbers in certain ways depending on your app. Light truck, easy race track, and/or short racing, you can drop the batt size down given enough C. If you have a hvy truck, long races, and/or aggressive driving, figure bigger batts/more amps/higher C etc.