I've been bitten by the vintage CDP movement and have acquired a few. There have been a few I've not been able to resurrect, and have been down the path you are taking.
The tda1541 and pcm56 are the two easiest to get boards for. The tda1540 (DAC board), I feel is coming, but may take a couple of years to show up on the market. There is a growing interest and momentum for this niche product.
For USB input you will need i2s, if the board doesn't have onboard connection.
And yes be prepared for casework gripes.