@Les i see you already in good hands
30q are a good choice. Legit source of cells are nkon.nl, but they stopped shipping to US just recently. The US guys can tell you better for a trusted seller overseas.
If you look into building more than one pack it might be worth investigating in a spotwelder like the kweld or malectrics, but than your 250$ budget will not be enough.
If you don’t want to build more packs in future you might be good with what @b264 suggested or you go with nese modules which make assembly pretty easy and no spotwelding is needed.
Important note if you get into building your packs, read through our battery builders club thread.
And don’t hesitate to ask your questions there. Upload pictures while you in process of building and we always can give you advice if we think you on a wrong way.
Also, make sure you get pure nickel! Saltwater test your nickel strips is mandatory