What are expl3 equivalents for various macros useful to debugging and a building private packages

You want \iow_term:n that expands macros in the usual way.

\RequirePackage{expl3}

\ExplSyntaxOn

\iow_term:n {Hi,~Andrew,~how~are~you?~This~message~will~automatically~
 wrap~when~reaching~a~predefined\c_space_tl limit. \\ \\
 You~can~force~a~break~and~use~\exp_not:N \tl_to_str:n
 for~showing~\tl_to_str:n{\whatever~you~\want}}

This will output

*************************************************
* Hi, Andrew, how are you? This message will automatically wrap when
* reaching a predefined limit.
* 
* You can force a break and use \tl_to_str:n for showing \whatever you \want 
*************************************************

So \iow_term:n is the analog of \typeout, while \tl_to_str:n is \detokenize; a space is \c_space_tl, which is exactly the same as \space.

There's also \iow_log:n for writing to the log file only.

grep is your friend:

l3kernel/l3tl.dtx:\cs_new_eq:NN \tl_to_str:n \etex_detokenize:D


l3kernel/l3tl.dtx:\tl_const:Nn \c_space_tl { ~ }

for messages if you don't like the \msg functions, probably \typeout is still your friend.