Plenty of good advice, so far, but I'll add a few specific points:
Brushes - I have tons of different shapes and sizes, but like ScootyPuffJunior, end up doing the vast majority of my work with two or three of them. Better to get a few quality brushes (and learn to take care of them! Look up The Master's brush soap - it's like magic, for this purpose) that you'll actually use. Aside from your go-to watercolor rounds (Kolinsky sable being the gold standard), you may want a stiffer flat or filbert for drybrushing, as well as a synthetic or two if you'll be working with harsher solvents or want a "beater" brush for spreading glue, filler, etc.
Plastic cutters/clippers - Reasonably nice to have, if you get a decent pair, but far from a necessity. A bit of care is required to actually get clean, flush cuts with them (contrary to advertisements and a surprising amount of internet scuttlebutt), as many sprues have attachment points so thick that the angle of the inner face of the jaws forces the waste away, resulting in tear-outs and distortion at the center of the cut. Best practice is to clip away from the part and clean up with a knife/abrasives (or even a second go with the clippers, as the waste now has somewhere to go) afterward.
Painting accessories - Nothing special about a water cup. Most people seem perfectly happy with disposable plastic (Solo cups and the like). The real item you want to focus on in this category is a palette. Anything non-absorbent, from plastic to metal to ceramic, will work, but I highly recommend whipping yourself up a wet palette, at some point. Plenty of success stories and DIY plans (so simple that I hesitate to even call them that) floating around, so I won't go into detail, here.
Files - A manicure emery board is quite handy, especially the multi-grit, foam-backed ones, but you're going to want some good files. Much like brushes, it's handy to have a variety for those rare tasks that demand it, but you'll likely find yourself reaching for the same few, over and over again, so don't think that you need a huge set to get the job done. Diamond grit needle files are pretty widely available, nowadays, but most of them are a bit coarse for my taste, leaving a rough, directionally scratched finish. They do, however, cut in any direction and without damaging the tool, which works out nicely for hobbyists unfamiliar (or just uncaring) about tools, who prefer to just scrub away at a mold line. I'm a big proponent of single-cut metal files, both for the rate at which they're capable of removing stock and the fine finish they leave (near mirror finish on metal, with decent files and proper technique). Some of mine are older than I am (decades old legacies of my dad's model shipbuilding and a good lesson in proper care and feeding of quality tools being the best practice), but I recently picked up a
set from Harbor Freight that pretty much floored me. The cut of the teeth isn't perfect - there are occasional shallow or bare spots - and many are double-cut, but after using them I'd easily have paid five times the $4 price tag. They aren't as tiny as some of the specialty modelling sets available, but I'd heartily recommend them as a starting point.
Saw - It's well worth getting yourself a razor saw, even if you work exclusively in plastic. Nothing will give you faster, cleaner, and
straighter (why, along with the fragility of the blades, I wouldn't recommend a jeweler's saw to a beginner) cuts through limbs for conversions, thick sprue, or heavier scratchbuilding materials. Good for use on metal and resin, too, which can dull knife/clipper blades and chip, respectively. Razor saw blades are frequently separable from the handles, which can also hold heavy-duty knife blades and gouges (both the X-Acto and Revell models are like this).
Pin vice - Not on your initial list, but an indispensable hobby tool, as others have mentioned. Along with some fine bits, it's great for drilling out gun barrels and vents, as well as for adding holes for pins or magnets during construction. If you add pins for assembly, you can also chuck those up and it can also serve as a handle for parts during painting. I highly recommend going for a single-ended model with a swivel back, for increased comfort, control, and potential pressure while drilling (double-ended jobs force your fingertips to do all the work). Interchangeable collets are also a must, if you plan to use more than a very narrow selection of bits.
Glue - I use both superglue and plastic cement (the solvent-welding type) regularly, as well as PVA for terrain and basing. Exactly
which I use at any given point depends both on the task and the material(s) being joined, but I consider all three types well worth having on hand. Also, glue is the one case where I would say DO NOT, under
any circumstances, buy from
GW. Most of their hobby products are at least decent, if overpriced. I've heard nothing but complaints, however, about their glues.
Putty/sculpting tools - Not exactly beginner stuff, but since it's been mentioned... Kneadatite (a.k.a. greenstuff/
GS), Milliput, and other epoxy putties are great for cleaning up joins and rough spots, filling gaps, and assorted sculpting/conversion work. They all have slightly different working and cured properties, so some hobbyists have favorites, change putties or different tasks, or even intermix them to tweak the behavior. For working them, much can be done with makeshift tools - bits of bent and polished wire, wet toothpicks and popsicle sticks, etc. - but a proper set of sculpting tools comes in handy. Squadron's set is popular for polished stainless tools, but I've recently discovered the joys of the color/clay shapers that Flinty mentions. I find the extra firm size 0 set to be quite nice to work with. While I still use the metal tools for some tasks, I find that I end up using a knife blade more often than them, with the color shapers doing the bulk of the work.
