Starting out, one can use a single-tray printer, or one can take the plunge for the additional $100-150 for a multi-tray. There are tools out there that will allow you to print one, then the other and provide a map to bring them together.
For example, there's Notary Rotary PDF Sorter. Also, SnapDocs allows you to download the documents broken-out.
As you move forward, you'll probably find the investment in a multi-tray printer, especially with packages over 100-200 pages long (often the most profitable).
My last few tray add-ons averaged around $200 in addition to the price of the printer. If your not sure whether a printer can take a second tray, google the spec sheet for the printer, and check the accessories entry - the tray would be listed there.
FYI, used multi-tray printers are available on ebay for $100-250 including shipping. These older printers also have the advantage of using non-manufacturer (third-party) toner.
One note: often one tray being 100-300 pgs; the other(s) something like 550 or 1050;choose which tray gets the legal by what your most common closings use). I've even played with a separator page (in blue) on a 3-tray printer.
As far as specific printers, I'm very fond of the Brother products. The Brother HL-L6200DW meets all the 'check boxes'... duplex, multiple trays, reasonable price ($150-200), good toner life (3k, 8k and 12k cartridge sizes) compatible with 3rd party toner, etc... Print cost is~$0.01/page for toner, even with the 12k OEM cartridge; good paper shopping puts you at around the $0.02-$0.025/page range (not including printer acquisition).
Additional trays are available - a 250-sheet tray (LT5500) and a 520-sheet tray (LT6500).
The Brother HL-L6200DWT includes the first additional 520-sheet tray (LT6500), saving about $40.
There are also many fine used printers and copiers; depending upon an NSA's other interests, investing in one of these may be indicated (I personally expect to buy a larger photocopier/printer for my tax practice).
Hope this all helps.