pretty much any semi stiff hide will work, like i mentioned above i have just been using skirting leather for all of it, so far it works fine, was some thick leather i had gotten from Tandy and didnt want to buy sole bends for learning, so far so good actually, even used it to make heel build up on a pair of logger/hiker work boots.
i did welted boots the first few pairs but have just been doing stitch down lately, just way faster and easier and just as durable, at least thats what i have found for myself. All my uppers have been from Nubuck or oiled leathers use veg tan calf for liners on most. I know a lot of folks get all uppedy with thread too, but ive been using 1.2mm tiger for stitching the uppers to the midsole. Used that for sewing the welting on first few pairs as well, super strong and resists rotting, linen thread not so much, again just something i decided to try.
happy shoe building!