I will third the Noga-style (swiveling hooked cutter) type debur tool. There are various types of tips, for different materials, right vs left vs either way cutting, pilot end (helps control scoring, chatter, and cut depth) and for inside vs outside (concave vs convex) edges. I have a couple of cheap 4 piece sets, and buy better cutters of the types I need. The linked on above is decent, and will be fine for most plastics. Practice, as there is a tendency for the edge to dig a little, based on the angle of the handle and position of the blade tip. If you look at the deburring tools catalog from Noga, you will be overwhelmed, but get a good idea of available tools.
I would also suggest a triangle (machinist) scraper. Again, for most plastics, any will do. They are inexpensive, as long as you are only concerned with softer, nonabrasive materials.
Files. They are inexpensive and are best bang for the buck in finishing. For cleaning outside edges, such as where tabs came off, an 8" medium (bastard), single cut flat american pattern, and a fine (second cut) in the same size, is a good start for acryilc. You can remove a fair bit of material and also draw file for dead smoothing. [Hand files are addictive. They are so useful–I have roughly 100 in regular use, and I have no clue how many in backup, but most are real specialty, like full runs of rifflers in several styles and sizes (0 or 00 to 6, depending). Don’t ask how much money I have invested, as I long since lost track… But I don’t have a problem. Really] You will also want handles. NEVER use a file without a handle. You may also consider other file shapes, and maybe a needle file set, if you will be doing through profile work (lacework, for example) or concave edges. [needle files often have handles molded on, as standard handles may be way too big and clunky]
I also second a sea-of-holes bed, but you can probably go a long way with just MDF wasteboards. A way to get repeatable positioning is to put holes in the wasteboard with the machine and insert dowel pins. I use 3mm dia, 10mm long pins for a lot of things so I always have them around, and they are cheap, so I tend to use those. Three pins is fine… two to locate one edge of the material, and a third to locate a second edge. Works fine with tape for hold down, as well.
With the sea of holes, you can predrill mounting holes and just screw the work down, as long as the holes can match the bed grid. You might need tape to help, as many materials tend to lift. Like any fixturing method, you need to let the path planner know where the clamps, screws, fixture parts are. For plastic on aluminum, the 3M 467 does well, but is a real pain to remove at times. It doesn’t take much to hold really, really well in this type of application. I like it because it is dead consistent in thickness, unlike many tapes [it goes on with no carrier. Just adhesive in a precise thickness] so if the bed is level, the workpiece is level. Handy when doing low depth operations like PC boards or surface engraving. Check for material compatibility before using adhesive if the hold-down face will be visible on the final product.
I would concur that a rotary tool (Dremel or similar) is really not going to do a whole lot in the application you are looking at. You may find it useful for pre-drilling mounting holes if you don’t have another drilling option.
Calipers are a definite tool to have. You will also want a way to set tool zero height. Many people use paper (bring the tool down until there is drag on the paper, then you are the paper thickness above the work), and it works well, for a lo of things. I often find it is awkward in this machine to position so I can feel the drag, so I generally use either brass or plastic shim stock (I keep 0.500mm plastic around for a number of things, and have a mystery mix of brass collected over the years) as the thickness is more precise and I can drop the tool until there is a visible scratch without damaging the tool. With sign making stock, paper is likely to be fine, as there will be little in your way, but there are other options if you need them.
You might also want a selection of abrasive papers (and holders if you don’t like wood blocks). If you want a matte edge finish, that is by far the easiest way. It is really tough to get consistent finish without a mandrel.
Ditto on a decent square. You can check a square easily to be sure it is actually square. A combination square is very handy. Avoid the cheap ones. They won’t be square or rigid. Bonus is that a combination square lets you measure 45 degree as well as right angles, and is also a rigid rule.
Tools for removing work from the machine, if you use adhesive,
If you go bed-of-holes, a selection of screws and washers for workholding. The selection needed will depend on the material thicknesses you work with. I have, at this point, maybe 30 or 40 screws for the Nomad in lengths that differ by 1 or 2mm. My preference is socket head, but that is due to the type of work I do and the setups I use. You might find a truss head more amenable (broader surface under a lower head). Or something else.
A work surface (cutting mat or vinyl drafting surface) is real handy, and a way to hold material rigidly during prep and finishing-- a vise, clamps and a table, it depend on the work. For what you are doing, a woodworker type vise mounted under a table, and soft jaw liners (leather is traditional for fine finish soft materials, but even craft felt will do for the type of light work you are looking at) would be my choice, but this a a really personal kind of thing.
Enough, already. I’ll stop.