Soft gray stuff might just be putty that they used as a dam for the epoxy. As everyone else has stated, it's not that bad of a job and if you screw up you just grind it back out and do it again :D
I've found great deals from Powder Valley on powder and bullets.....I normally wait until there is a free hazmat promo going or as soon as a powder comes back in stock if it's a hard one to get.
I just got done working up a hunting load for my wife's 300WSM with 24" barrel.
Brass: Winchester fully prepped
Primer: WLRM
Bullet: Hornady 178gr ELD-X
Powder: W760 63.0gr (node 62.8-63.4)
Groups right at 1/2 MOA
Should be able to find an AICS pattern single stack 5 rnd mag that'll work out real slick. They normally have 3 long action COAL lengths to choose from 3.715 up to 3.850 (if memory serves right.