The scale is usually shown as a scale bar, but there are many inventive ways of depicting such a thing, which you will discover as you browse through the many beautiful maps on display in the
Finished Maps forum. Another good place to look at great maps is on the
Cartographer's Choice Forum.
GIMP and Krita are both free, and there are other software packages as well. I only have a few hours experience of both, so I'm not really the person to ask for advice, but there are many very beautiful and instructive tutorials available in the
Tutorials Forum, and of course there is also a wealth of information available on YouTube.
As for which tablet to buy I can't really say what would be best for you, since
the one I am saving up for is one that was recommended for me personally by ChickPea as being the best value for money given a £50 budget, and may not necessarily be the best buy for you. There are two basic types, as far as I can see - the ones that take the basic form and shape of a giant pressure sensitive laptop touchpad (like the one I intend to buy), and the more expensive ones (which can be over £1000) where you draw directly onto a screen displaying your artwork. I guess its like all things - "you pays your money, and you takes your choice"