Hi Dan,
The laser engraver isn't going to be sufficient for producing the optic grooves for specular holograms. You're going to want a rigid CNC - and the larger the tabletop type CNC the less rigid it will be (unless it's designed to be large *and* rigid). The reason is that it's difficult to get optic grooves to form that aren't wobbly if the machine isn't as rigid as possible. You want to use something like a diamond-drag bit, or a carefully rounded carbide point that you can score your plastic or aluminum with to produce optics. Plus, if you get a CNC you can make other stuff too! A laser engraver will only be able to do so much, by comparison, even though it can do some things that a CNC router/mill wouldn't be able to do - or do easily.
The minimum depth optics you can produce can easily be 0.001-0.003" deep but the real question is how wide they are. The wider the optics the less you can fit into your hologram before they start overlapping and degrading one-another whenever they find themselves densely packed into an area. It's a fine balance between minimizing the number of optics to reduce interference (it's OK if you have some that intersect eachother but not a whole bunch cross-crossing all over the place which ruins their ability to reflect the light source) while also having enough optics to convey the geometry in question.
Watch out for creating holograms will too many optics - it can take a long time for Holocraft to output your SVG/G-code (especially if you have occlusion detection enabled) so you want either simple geometry or to take a complex model and devise a way to visually convey it to viewers using the fewest number of optics possible. It's very easy to over-do it.
The other trick is making the hologram as big as possible - which means having a machine that's big enough to fit your hologram material into that you want to score the groove optics into while not being so big and flimsy that the optics come out all wobbly. Playing with feed rate and adjusting cutter diameter to control the groove depth can have a big impact on whether or not resonant frequencies are generated while optics are formed. Optics also come out more reflective if you run your machine as fast as possible - trying to mitigate the speed wobbles by running it with a slow feed rate will not only take forever to produce a hologram but also produce grooves that are not very optically reflective.