Given the amps you want, and the price,, you are pretty much looking at lipo.
As always,, store it and charge it in a place you would build a fire. I'm not saying lipo will burn,, just that any battery type that big makes a big fire if it does burn. Naked pouch batteries like lipo have the additional risk of burning, if they get dinged, or you run a really puffed cell.
As for Hobby King,, my luck has been ok lately. I don't know about multistars,, I have bought turnigy and zippy in 20c and 30c versions. I'd recommend turnigy, in the higher 30c or better configuration. Those will come with big fat wire, and high current 5 mm connectors. You can start with as little as 10 ah of that for 50 amps of pull, then add more 10 ah at a time till you have a tolerable level of sag under load.
As for how to connect up,, well there are a shitload of ways to do it, all right. That's the beauty of lipos. When building bigger packs, I liked to test packs, then when they are proven ok after a few cycles, then parallel packs to make 10 ah blocks out of paired packs, whatever type they may be.
Clip both packs discharge wires, then connect to one set of the large bullets. I make the connection with a thing called a crimp sleeve. I got those in the wiring section of a builders supply. Very easy to crimp, resulting in a fairly neat connection for 10 ah. Half the connectors helps with larger assemblies of lipo.
Then to get to pack voltage, simply string in series for a 10 ah, whatever voltage, pack. Nice neat wiring. I personally do not parallel at the balance taps. I just use a 1s charger to bring up low cells individually when I balance a big pack. Check them individually often, with a cellog.
To make larger than 10 ah,, I then string full voltage packs in parallel,, to do that make an octopus connector with big fat wire, like 10 g or larger. One end to the controller, then for 40 ah, four sets of connectors on the other end. Your packs can then be spread out around the vehicle as needed, instead of trying to mount a gigantic 72v 40 ah brick. Each 72v 10 ah section will be about small shoe box size, and weigh about 12 pounds.
So,, how do you monitor the voltage of that many cells, for knowing when to stop? You don't. You figure out which pack in your entire assembly has the lowest capacity.
Then you put one of those cheap lv alarms on that pack. Only for a ride you know will flirt with lvc,, not permanent. When that weak pack is empty, the whole pack is done.
