I agree with Colin; NCE decoders are priced and set up nicely. I have used NCE decoders on two of my AM SD60s and my SHS switcher. I have two GP35 BNs, #2507 with Sound and #2521 B-unit without sound.
I installed the TSU-AT1000 Part#828039 EMD567D Turbo sound decoder. I paid $68 for mine. It works and sounds beautiful. I used Miniatronics LEDs with resistors and Soundtraxx speaker. I built my own speaker box and styrene frame for the speaker and decoder to mount to.
For 2521 which had some hand rail damage I put in a budget non-sound decoder from Sound Trax to possibly better speed match to 2507.
MC2H104P9 Part# 852004 I paid maybe $22 for it. I think the wire connections are a lot more durable than NCEs but both work great.
Good Luck, if you are on FaceBook I can send you some pictures.
John Ham