Give the Diamond Memory Lane Jr a try. It’s not as old school analog sounding, but it’s a really great delay. The Quantum Leap is also quite nice, and offers several weirder sounds as well which could be very useful.
That said, the EHX DMM XO isn’t bad at all. There’s a difference, but it’s pretty subtle, and I doubt you’d be able to differentiable the two when you’re creating ambient sounds. Plus it’s smaller and more reliable.
Of course, Ed made weird amazing ambient textures long before he had any rare analog boxes with modulation.
One of Ed’s favorite delay pedals is the Boss DD5, and you can’t go wrong with it. I’d also give the DD7 a shot, since it offers kill-dry reverse delay which DD5 lacks, as well as a fairly good emulation of the Boss DM2 (a cool, darker analog delay) as well as a modulated digital delay.
There’s also the EHX Memory Man with Hazarai. It lacks the modulation sounds of the DMM, but offers many more delay options.
And of course, there’s always the Line 6 DL4, or alternately the M5 or Echo Park (the M5 will sound just like the DL4, while the algorithms were tweaked slightly for the Echo Park so there could be subtle differences). These pedals offer a ton of delay sounds, both analog and tape-emulating, digital, and some weirder options in the case of the DL4 and M5. The DL4 also offers a powerful looper (as used by Ed for some very cool sounds on Little by Little).
If you think you may end up getting three delay pedals at some point in the future, then you could get one from each “group,” such as one DMM XO as well as a Memory Man w/ Hazarai. If not, pedals such as the DD7 and DL4 will give you more options than a single simpler delay would, even if they don’t sound ~quite~ as “good.” Of course, since you’re making ambient sounds, those differences will be obscured.
All said, you really can’t go wrong with any of the pedals that I’ve mentioned, so you should watch videos and try all of them if you can. See which one inspires you!