No need for the neck glue - the JMA-1L is a bolt on neck, so you don't glue it on.

In terms of sandpaper - you'll only use the 800 1000 1200 1500 2000 2500 3000 for a cut and polish at the end. In general, the grits below 600 should be fine for the body to get it up and running. Given you're spraying the body and its ash - you might want to consider a filler - something like:
https://www.amazon.com.au/Filler-Wat...rds=timbermate
I'm not sure what is available in France - if you can get Timbermate for a reasonable price, its a good product.