Though there's a lot to be said for the old HP plotters, if eBay is any indication, there's a very thin supply and any uptick in interest is going to cause them to dry up and prices to increase. It looks like that may have already happened.
One alternative for people who just want to get started today is to buy a modern version: they don't sell them as "pen plotters" exactly, anymore, but they do sell desktop vinyl and paper cutters (sometimes a/k/a "die cutters", although that's an inaccurate name used by papercrafters for historical reasons) that are very, very close in terms of operation to a 1980s moving-media pen plotter. And most of them can be trivially adapted to use a pen or marker in place of a cutter blade.
I have a "Silhouette" brand cutting plotter, which can take two tools, and it cost me about the same as what used 7475As are currently selling for and worked via USB out of the box. It'll never challenge the old HP on its great 1980s aesthetics, and the paper traversal speed isn't quite as fast, but it produces output that seems equivalent and I can jam pretty much any type of pen I want into its toolholder. It uses a variant of HPGL that's supported by several open source tools in addition to the OEM software.
The one important note is that you must absolutely avoid "Cricut" brand cutting plotters, which are unfortunately the dominant modern manufacturer. They have an awful razors-and-blades business model that revolves around selling the machines at a discount and then charging users for overpriced vector drawings through an "app store" type system, and they enforce this by obfuscating the communication protocol used by the plotters, and abusing the DMCA and engaging in other bits of sharp practice to discourage 3rd-party software. Unless you're buying one to reverse-engineer, they are best avoided.
The Roland STIKA line is very well-regarded if you want to spend a bit more. Can't speak for many others.
I'm hopeful that the current resurgence of interest in plotters will result in more third-party, open-source software for these current-production commercial machines, in addition to the older surplus ones. Once the parts supplies dry up for the older machines, these newer ones will be the easier path forward for people who want to start playing with this technology.