The Perfect Setup to hack on Guix is basically the perfect setup used for Guile hacking (see Using Guile in Emacs in Guile Reference Manual). First, you need more than an editor, you need Emacs, empowered by the wonderful Geiser.
Geiser allows for interactive and incremental development from within Emacs: code compilation and evaluation from within buffers, access to on-line documentation (docstrings), context-sensitive completion, M-. to jump to an object definition, a REPL to try out your code, and more (see Introduction in Geiser User Manual). For convenient Guix development, make sure to augment Guile’s load path so that it finds source files from your checkout:
;; Assuming the Guix checkout is in ~/src/guix. (with-eval-after-load 'geiser-guile (add-to-list 'geiser-guile-load-path "~/src/guix"))
To actually edit the code, Emacs already has a neat Scheme mode. But in addition to that, you must not miss Paredit. It provides facilities to directly operate on the syntax tree, such as raising an s-expression or wrapping it, swallowing or rejecting the following s-expression, etc.