Spot the loaded question!!
May as well ask "How Long is a piece of string" as to ask what is needed for transfers.
If we can make the assumption you are referring to transferring old Quad material, then you'll need (obviously) the means to output from your Quad setup into a computer.
So - let's look at some options.
1 - SQ Vinyl (perhaps the easiest) will require either a decoder and 4 outputs into a soundcard, and from there into a multitrack DAW environment. Alternatively, record the intact SQ matrix as a stereo 24/96 (or even a 24/48) WAV file & decode to 4 channels using the Adobe Audition scripts here in these forums. I can attest to these working very well indeed.
2 - CD-4 vinyl is much harder - you will need a properly set up turntable & demodulator. I have never yet heard anything better than Cai's work in this format.
3 - Q8/Q4 requires the correct tape decks, and decent copies of the tapes. For Q8, you won't beat what Winopener is achieving - the fidelity is incredible & at times you completely forget you are actually listening to one of the most fragile formats ever developed.
Once you get the files into the computer, a multitrack system is needed for outputting the 4 (or 6) mono WAV files.
Again, Adobe's Audition will do this, as will Sony's Soundforge or Vegas, or Steinberg's WaveLab/Nuendo/Cubase.
Output as 24/48 or 24/96 WAV files.
Authoring to DVD-A is the tricky part as there are many options.
Cheapest & best value for basic stuff is Cirlinca's DVD-Audio Solo at $45. Very basic - no custom menus, no MLP support (so no 5.1 at 24/96 or 24/88.2, as you must use MLP for these resolutions) & no bells or whistles at all.
Feed it the correctly named WAV files and burn to disc. Couldn't be easier.
Steinberg's WaveLab 5 or 6 will also handle authoring with basic custom menu design options. It's clunky, but once you get used to it it does work.
Top end tools include Minnetonka's DiscWelder Chrome - but this is very buggy indeed, and has some nasty issues for the unwary (MLP support, but no track points in Long MLP with custom menu designs including browseable images without glitching audio for one, and no warnings at all if you go out of spec as it is an abstraction layer tool - all that happens if you go off-spec is that you are unable to do what you think you should be able to do. It does include adding Video_TS, but the AutoMirror option is Dolby Digital and no way to pick streams from the menu system unless you reset to VIDEO mode in the player. Imported Video_TS will not allow independant selection of multiple streams either, so if your Video_TS carries DD & DTS, you cannot pick the stream - it defaults to stream #1. PGC Blocking does not work reliably either, so mixing stereo/surround (a spec requirement) is not easy to do and will usually require a separate group for each stream).
Sonic's DVD-Audio Creator is the best option, but it is brutally expensive. It is the only app that will do commercial grade authoring.
For personal use though, any of the above will get you where you want to be.
Adobe's Audition 2 came with a trial copy of Minnetonka's Entry-level "Bronze" application - think Cirlinca, but with an easier interface.
It really is dependant on how deeply you want to go into this. In single group Quad transfers, one is pretty much as good as another.