% Example and test for the substitutefont package:
\documentclass[a4paper,DIV14]{scrartcl}
\usepackage{parskip}
\usepackage[LGR,T1]{fontenc}
\usepackage[greek,english]{babel}
\usepackage[colorlinks=true,linkcolor=blue,urlcolor=blue]{hyperref}
\usepackage{bookmark}
\pagestyle{empty}

% \usepackage{substitutefont} % obsolete

% Serif
\usepackage[sc,slantedGreek]{mathpazo}
\DeclareFontFamilySubstitution{LGR}{\rmdefault}{udidot}

% Sans
\usepackage[scale=.90]{tgheros}
\DeclareFontFamilySubstitution{LGR}{\sfdefault}{neohellenic}

% Monospaced
% \usepackage[matchlowercase]{tgcursor}
\renewcommand{\ttdefault}{txtt}
\DeclareFontFamilySubstitution{LGR}{\ttdefault}{cmtt}  % CB fonts
% \DeclareFontFamilySubstitution{LGR}{\ttdefault}{fdm}  % Droid Sans Mono


% Teststring
\newcommand{\alphabet}{%
  abcdefghjiklmnoprqrstuvwxyzäöüß
  \foreignlanguage{greek}{abgdezhjiklmnxoprsctufqyw} \\
  ABCDEFGHJIKLMNOPRQRSTUVWXYZÄÖÜ
  \foreignlanguage{greek}{ABGDEZHJIKLMNXOPRSTUFQYW}
}

\begin{document}

\section*{Palatino/Helvetica/TXTT and Greek replacements}

Default font families after loading mathpazo, tgheros, and tgcursor:\\
familydefault: \familydefault{}, 
rmdefault: \rmdefault{},
sfdefault: \sfdefault{},
ttdefault: \ttdefault{}

Palatino (mathpazo) with GFS Didot:

\begin{description}
  \item [Normal:] \alphabet

  \item [it:] \textit{\alphabet}

  \item [sl:] \textsl{\alphabet}

  \item [bf:] \textbf{\alphabet}

  \item [bf it:] \textbf{\textit\alphabet}

  \item [sc:] \textsc{\alphabet}

  % \item [bf sc:] \textsc{\textbf\alphabet}

\end{description}

Helvetica (TG Heros) with GFS Neohellenic:

\begin{description}

  \item [sf:] \textsf{\alphabet}

  \item [sf bf:] \textsf{\textbf\alphabet}

  \item [sf sl:] \textsf{\textsl\alphabet}

  \item [sf sc:] \textsf{\textsc\alphabet}

\end{description}

There is no Greek extension for the monospaced standard PS font Curier. 
A good replacement are the teletype fonts from 
\href{https://ctan.org/pkg/newtx}{txfonts} and 
\href{https://ctan.org/pkg/cbgreek-complete}{CB-Greek}:

\begin{description}

  \item [tt:] \texttt{\alphabet}
  % \item [tt bf:] \texttt{\textbf\alphabet} % not in CB-Greek
  \item [tt it:] \texttt{\textit\alphabet}

\end{description}

\end{document}
