D series and turbo is the must cost effective power period. Build parts, replacement parts, and so on are so cheap. Plus a slight build will allow you to make more power than you can probably put to good use on the street.
If you went with the LS, i wouldn't swap heads, and just I'd just build it for boost. I know I'm old, so i remember guys like "cheesefrog" with a crazy fast ek hatch that was just a built LS turbo hanging with a dodge viper. If that isn't enough daily drivable power.........then you need a K lol