[HECnet] PDP Ignorance

OK Johnny, So to not appear too one tracked, and maybe draw the subject a little more towards your heart ;) Since my recent acquisition of various PDP handbooks I've decided to delve the world of PDPs a little now I've been able to read up on the (somewhat confusing) model numbers. Could I have recommendations of where to go in terms of an emulator (assume linux based), which model and operating system? What I'd like to do is play with some of the languages, especially but not exclusively APL, I don't know what software there is out there, whether licenses are required, etc. I'm guessing this is all very much before the SPL started... Thanks for the help, Mark.