Considering that you are using this for a high power draw situation, I would suggest avoiding the LiIon 18650 type cells and go with LiFePO4 chemistry. It's great for high current and safety. In the past I think you could have harvested A123 cells from power tool packs (Dewalt brand rings a bell), but of course they have all switched to LiPo packs as they can be made to fit compact and oddly shaped spaces.
The problem with large "packs" of 18650 LiIon cells is, that if a single cell goes bad and enters a thermal runaway, it will likely take the rest of the pack with it. It does not take a large amount of imbalance to "get in trouble" with LiIon. A "Vent with Flame" episode can be nasty. LiFePO4 is more forgiving of abuse than LiIon and LiPo, something you would appreciate if it should ever happen.
Parallel charging cells at a slow rate using CC/CV would help minimize the need to balance charge, but discharging them in parallel, as mentioned earlier in this thread, would still allow a weak cell to to cause an imbalance. Considering how much variance there is in the capacities of 18650 cells from the same lot, I would not risk it.
This sort of thing "parallel discharging" etc. is being done today, but primarily by remote control aircraft hobbyists, who are acutely aware of the risks, and who take great care in maintaining their battery packs, including measuring imbalances/discrepancies in each cell of each pack of cells. Well.... that or watch their planes go up in smoke.
I've looked at some of the battery packs from Golden Motors Canada, the prices are not for the faint of heart, but personally, my hind end (not to mention the garage or house) is worth more than the few dollars saved by trying to go cheap. I only mention Golden Motors as I have perused their website. I have no idea of the quality of their products, and I'm sure there are plenty more suppliers now. I only remember them from a couple years ago when I was toying with the idea of an ebike for getting to work.
Regards
Christian