I only really use dissecting scopes for dissecting verts, and for this, the quality/power doesn’t need to be great. For inverts, I would consider what magnifications you would want (most scopes have two options, but the actual power can differ).
If using it a lot, the two most important things for me are the adjustability of the eyepieces and the lighting. For eyepieces some scopes just work and some are uncomfortable to me. I haven’t found any rhyme or reason, so maybe try before you buy. For lighting, you can probably jury-rig something, but there are handy light rings you can use that do a good job and reduce eyestrain (I haven’t bought one myself, just used them).
Other big consideration is whether you want to take a lot of pics or not. If not, the old cell phone or pocket cam to the eyepiece can work for occasional semi-decent pics. If you want regular pics, getting a cam attachment will be worth it.
The old dissecting scopes (from like the 40’s or whatever) still are often solid, so if you see used and only need something basic, that may be a good route.