Here are your parts
Replacement parts for WHIRLPOOL DUL300XTLT2 Undercounter Dishwasher | AppliancePartsPros.com

Here is the tech sheet
http://www.servicematters.com/d...0Sheet%20-%208537797.pdf

A repairman has told me that a new control panel
Unless the repairman actually did some checks/tests he is making a guess.
The problem you describe in your other post can be caused by either the control board or the control panel (keypad).
Try the below cleaning procedure first.
Try running diagnostics.
If it does nothing, check the switches on the control panel with a meter.
If the panel checks out OK then odds are that it is a control board problem. Still a bit of a crap shoot but you have improved your odds of replacing the correct part.

Can anyone direct me to a much more reasonably priced source for part number 8534838, either new or used?
I do not know of a lower cost alternative other than using a white one.
I would not recommend installing a used panel.
At $400.00 it makes more sense to replace the unit.
Even at $150.00 I would not replace the panel unless the unit is in very good shape.

Connector Cleaning / Checking
Unplug the unit
Open it up
Remove the ribbon cable from the keypad to the control board at the control board.
Clean the edge connector.
If it is the type where there is not a connector just the insulation is removed, it can be cleaned with a pencil eraser. Do not use an ink eraser as it is too abrasive.
Check the tracks in the ribbon cable for hairline cracks. A magnifying glass helps here.
Would not hurt to clean the connector on the board. Use electronics contact cleaner or isopropyl alcohol and a tooth brush. Be gentle and do not reconnect till all the solvent evaporates.
Reconnect everything and give it a try.