I used the EZ UP adhesive remover from Lowes and it worked fine. You'll only need to do this if you are relaying glue down carpet, the stretch carpet uses the tack strips.
The leveler you referenced is a self leveler and is just mixed and poured and levels out the low spots. There are two types, make sure you get teh self leveler unless you want to level it out yourself. Just read the package instructions on mixing and pouring.
Scraping off the old glue isn't hard, except on the knees and backs. Just make sure to let the remover do it's job breaking down the glue before you start scraping.