Yep, the reviewer's guide would be a good start; also read the Extreme Installers article, its a nice walkthrough.
You're a lot better of re-writing your custom actions in MSIcode. You will see that what took, say 10 lines of C++ code, will most often be 1-2 lines of MSIcode in your new script (the built-in functions help with that). You can also call arbitrary DLL functions and run EXE files from MSIcode, so that should help with the migration.
Correct about Admin Studio - and, there is also the redistributable automation interface (if you don't immediately recognize what that is, chances are you dont need it
).
