Ambient loops are my daily bread, and as an impoverished creature I've generally achieved perfectly satisfying results with just a dedicated, versatile reverb and any ol' looper. I really can't imagine ambient without reverb- it gives you that deep, profound sense of space and sonic comfort.
To suit my purpose, I use a Boss RV-6, which is a relatively inexpensive but hecka deep multifunctional reverb. Does all the basics- plate, spring, hall, whatever- with a few special tricks up its sleeve, including two I think are just totally jack dandy for ambient: modulated reverb, which just sounds magical on the ears, and "shimmer," which essentially adds an ever-ascending pitch shift to your reverb that sounds like you're being joined by a chorus of angels.
On the higher end of the spectrum you'll find the Strymon BlueSky and BigSky, which cost twice and three times as much, respectively, and do what the RV6 does while sounding so beautiful your ears may cry (doctors cannot yet explain this phenomenon and are very frightened), while on the lower end the Mooer SkyVerb is an adorably solid pocket-sized dedicated 'verb that doesn't even try to hide how much it wants to be a BlueSky when it grows up, and will run you maybe two tanks of gas used, depending on your vehicle.
Can't really go wrong with any of 'em, and I'm sure there are about twelve hundred other good options out there too, but these are my favorites, at least.
--^@