The A1139 variant is the "best", and most powerful G4 laptop. It has a 17" 1680x1050 LCD, can support up to 2GB of RAM, has an ATI Mobility Radeon 9700, and comes with a 5400RPM HDD up to 120GB.
You can put in a KingSpec PATA SSD, or you can get a PATA-SATA adapter that will allow you to have a mSATA SSD. Presumably any mSATA SSD should work that way, but some people have had issues with SSD's larger than 128GB. YMMV
I think it has a ExpressCard slot as well, so you can add extras like USB 3.0(though that will require a extra power bank if your devices will draw more than ~500mA), if you can find drivers for them(the drivers probably don't work for 10.5 Leopard, but Linux should recognize the cards). There are also 802.11N ExpressCard add-on's as well, you might look into those if you need better wireless capability.
The highest macOS that these can run is OS X 10.5.8 Leopard, there are many sites where you can find programs that still work on Leopard. Some Linux distro's are still supported, like Debian and Ubuntu MATE.