How to Pick a Build
Builds depend on three rolls in the first floor: your starting weapon, the spell offer at the first shrine, and which joker pair the first vendor sells. If two of those three lean DPS, commit to Crit Sniper. If two lean defensive, go Frontline. Don't force a build that the run doesn't give you.
Upgrade Path
See Best Upgrades for which permanent upgrades to buy first. Short answer: Reload Speed → Magazine → Crit Chance.