You buy a set of 4S hardcase brick pairs to get a 10Ah battery. For lower powered builds you easily get away with single 5Ah bricks in series, so no paralleling of packs needed. Just connect the discharge leads together, easy as that. But then you get into trouble of monitoring the cell voltages all the time, making sure you know a cell goes wrong before it actually makes the battery unsafe. You get used to that, and a 45V 10Ah battery may cost you as little as 200 bucks, and weigh just over 3 kilos, or just under 5 for a 15Ah pack, but you'll need a bulk charger, lipo buzzer and a battery medic at the very least. I can advise where to get all these, but you must first make the decision yourself. Later, as your demand for speed grows, you easily add more bricks in series to get more voltage. You can't do this to any commercial battery since it's not so modular and its BMS won't allow for more cells.
There is also a BMS available from BMSBattery site for $25 that has 20A constant to 40A peak power and can work with 12S lipo, and I tested one briefly, but can't really tell you if it's reliable in the long run. There are other offerings within ES that are more expensive, and better quality it that's what you're after, but I started with lipo as my first build, never had a BMS, and never regretted that. If you're good at soldering, watch your battery cell voltages every use, understand the ohms law and its consequences, and keep your lipo from flammable objects and physical damage, you'll be fine.
But, all of this may be over your head. It may be double the price, but going for A123 or Headway LiFePO4 packs with BMS may pay you off later.
