Do I need a DAC for my Marantz SA14

Not neccessarily upsampling.
Upsampling only complicates the overall proccessing and increases the digital noise indeed. A good quality DAC may do the trick but you can get quite good one-box player for the descent price such as Cambridge D500SE demo used or new either one won't break a bank.

Not necessarily upsampling, but you can improve on the SA-14 redbook. Yesterday I hooked up my headphone amp, which is normally connected to my Marantz SA-14, to my EMC-1 and the difference was dramatic. The EMC-1 was far better. You might check into the EDC-1 DAC which does in fact upsample.

The Musical Fidelity A3 24 is very marginally better, and personally I liked the MF better overall compared to the Bel Canto, Chord and MSB dacs, fwiw. I sold the MF dac when I bought the Marants SA-14 as I felt I needed 100 more cd/sacds more then I needed a minor, though apparent, improvement in redbook playback (I can give specifics if you like). If you have the loot for a top notch dac, sure, you can surpass the redbook of the Marantz. The SA-14 makes for a great transport as well.

However, before adding a dac I'd spend time with isolation devices, power cables and interconnects. This is a very tweaky player, especially in terms of isolation, which will change it's character dramatically, more so then the addition of any of the above mentioned dacs ever would.