I purchased some of those LJ 4M+ back when I did IT purchasing for the agency I worked at. They're great. After one adds the duplexor they are still modular enough to take a 500 sheet paper tray. I'm not sure how high you can stack the thing up, but you can add the duplex unit under the printer and then at least one, but perhaps several (4?) 500 sheet trays under that.

The Postscript SIMM should be available on Ebay at a "make it worth my while to put it in a box" price, I think. The other SIMM slots will accept most 72 pin RAM, although there are some PID pins which could conceivably cause issues. HP always claimed that only their RAM worked in HP printers, but in this era, when their RAM was standard 72 pin SIMMs, the only RAM I ever had trouble with was authentic HP RAM. All the 3rd party stuff I tried worked fine, but their expensive RAM wasn't recognized in the PaintJet XL300 (not sure I remembered that model right).

Let's see, for fun, this is also the printer recommended for use in the Toner Transfer Method of printed circuit board etching:

http://www.pulsarprofx.com/pcbfx/main_site/pages/products/transfer_paper.html