The DCD-1520 replaced the DCD-1500. I cannot attest to the build quality or sound quality of the 1520, only the 1500. The knob on the front would control the headphone volume, but not the output level from the variable output on the rear RCA jacks. If the 1520 is is like the 1500, it has both fixed and variable outputs, but to control the variable outputs you need to use the remote.
The 1500 has a digital output port, but being one of the first players to offer this feature, it uses a non-standard connector. I believe the 1520 may have a more standard output port, important if you ever want to use an external D/A converter.
The 1800 was Denon's earlier, top of the line, CD player. The DCD-1800R is contemporary with the DCD-1500 and has similar if not identical features, slightly better specifications, and a larger power supply. The power supply of the 1800R actually extends the rear panel of the player a few inches, making it a bit deeper than the 1500. Both the 1800 and 1800R are less common than the 1500 or 1520. There is also a model 1500 II, this model may have the optical digital output port.
IMHO, I would pass on the 1800 as it is a 1st generation player. I would also pass on any player where the mechanism was not operating properly, unless you are willing to repair the unit or pay someone to do it. The stuck drawer is probably a slipping belt, but could be the motor that drives this mechanism. I'd pass on this one.
Another series to consider are the Magnavox single disk CD players (made by Phillips) that feature Dual 16 bit converter technology. The model 482 is fairly common and inexpensive and sounds very good, similar to the Denon although the Denon put out more bass.
