Only since you are a noob, you need to know that converting analog DC locos to DCC will cost in the vicinity of $45 per, finding the best space to mount the circuit board, securing it in place, then wiring it all in, is no cake walk. `And if you want sound, mounting the speaker is yet another tough hurdle.., not to mention that early open-frame DC motors are not that great for DCC.
Next: Do you want to have operating knuckle couplers such as Kadee or McHenry ? Because I sense many of your cars have the old, archaic 'horn hook's on them; do you plan to replace them with knuckle types ? If so, that's another huge hurdle..
Next: Many older loco issues have deep flanges on their wheels. If yours do they will likely hit the simulated spike heads on code 83 rail..
This is not necessarily the views of all non-noobs. But when considering all that, personally I would mothball most the engines and cars and start anew with one good, modern DCC/'sound on board' loco and a few new cars, using code 83 or code 70 track, and put my energy into the best construction method for my benchwork..Far as a controller goes, I have an NCE PowerCab and very happy with it ($150 ish)...
Please do realise I'm not berating you for your wishes. I'm just giving you a heads up on what you could be getting involved with...M