I would say:
1. Save for a while longer and get a pre-made battery from Ebikes.ca, or ping, or EM3ev, or Luna.
2. Get a smaller battery and know you will need to pedal more to get longer distances.
If you build your own pack, you will need to buy batteries, perhaps a bms, a $100 to $200 tab welder, nickle strips, shrink wrap, wires and maybe plastic of fiberglass board to keep it all protected. Then you have to take the time to research and learn to properly build it.
Now, you could try to find an old 1000w or larger microwave oven and harvest the coil and learn how to properly build a tab welder from that. If you have that transformer, an old set of car battery jumper cables and some simple tools you could build a crude tab welder.
You might be able to use a 65 to 80 amp car battery to build a crude tab welder as well.
Any of this put together will cost you about as much as that extra $200 you would spend over you battery budget as it stands.
The cheapest way to try to build a battery is to get a second hand 100w soldering iron and solder the pack together, but you also run the risk of melting the internal plastics in the cells so you degrade, or ruin a cell and then have to figure out which one is the problem.
Kits are available for 18650 batteries, but they cost money as well.
I would just take a deep breath and know that you need to save that extra $200 if you want to get a good battery.
Favorite Quote: "This is L.A., sugar. There is no 'over the top." --- Chris Erskine
Current build: Liahona w/ cheap front suspension and suspension seat post. Yescomusa 36v 800w generic front hub motor. 15ah Headway triangle mounted pack. Tronsung 30 amp,
Previous Build:1992 Trek Antelope 800 - Bone Crusher (no suspension) - Yescomusa 800 watt 36 volt front wheel kit. Don't do it! Get suspension!!!