There is a window now where you can actually piece together a fairly high spec, low latency PC build at a reasonable budget.
RME stopped making these analog output expansion boards. They don't work with the latest RME HDSPe AIO pro cards, only the older HDSPe AIO and HDSP 9652. It is a smart forward-facing move, but it reduces the functionality if you wanted to use your system as a multitrack recording/tracking system to replace an analog one in an otherwise analog studio setup.
Right now the older HDSP and HDSPe cards that these work with still outperform external port interfaces, and now they available for a fraction of their original high price tag. Of course the newest RME stuff has even lower latencies, but it's RME so it also costs their a high premium for new current hardware.
RIght now is this window where the older gen RME hardware, which is still near the top, is available for a fraction of its original cost. Up to three cards can work on one system, and all can have one additional input and output expansion board apiece.