You can do significant editing of tracks on these machines without using a PC. You can do a great deal right from the front panel/remote. The editing functions are supposed to be a superset of those included on the D1624/D824 family... You can scrub to locate edit points, cut, copy, paste, repeat paste (to build looplike tracks)...
Mixing and mastering are another item: the Fostex D2424/D1624/D824 recorders are digital multitracks, _period_. To mix down, you need an external mixing board (either analog, digital, or DAW). They literally are tapeless mutitrack tape machines: they record and reproduce multiple tracks in parallel, and that's it.
I did a lot of work with mine with just my analog mixing board before I built my DAW. Even today, I primarily use my DAW for backup (flying tracks over from my D1624 via lightpipe, and archiving them on my network), and mix all in the analog domain.
The DAW is a nice companion for the D-series boxes, but it is not even close to a requirement- assuming that you have an analog mixer.