As for LXDE consuming huge amounts of memory, it depends on your definition of huge :-)
Very true. My test machine is an old dual processor PIII with 1 GB of memory and It is clearly underpowered for Zentyal 3, so "huge" is easier to achieve
. Keep in mind that you can log completely out of the local Zentyal desktop when not in use, which significantly reduces (to an acceptable level even on my test machine, from my point of view) resource consumption from that direction.
Also keep in mind that even on an underpowered machine, Zentyal performs better in production than during configuration. It takes me
forever to do an update on my 3.0 test machine--several minutes from start to finish--yet it is pretty responsive when in actual use. The single most important goal, as christian noted, is to avoid swapping while the machine is in production. Given your deployment goals and 1 GB of RAM, you might find it doable.