$40 is a great price for a (working) mac plus and keyboard. Mac Pluses are actually my favorite old mac. just enough ram (4 MB!), super fast startup (using System 6), and some decent applications. I used one for writing for a while, then remembered that writing is HARD. But that wasnt' the mac's fault.
$40 for the keyboard alone... not sure what a plus keyboard goes for these days. If you were local, I'd give you one of mine, a guy gave me a horde of them a few years back.
I'm personally not crazy about the switches in the mac plus, I prefer the 128k/512 M0110 switches, but then that keyboard is pretty 'stiff'. (and I think most people who have used both prefer the Mac Plus keyboard).
Anyway, it's a fun little project.