If you're going to spend money and not purchase a combo I would recommend spending more money on the rod than the reel. I find having a better rod is much more beneficial than an expensive reel.
ive used a 30 dollar piece of crap okuma slv reel my one handed and my 2 handed setup for both steelhead and salmon for 3 years now without any issues. when my drag starts to peel a bit too easy its simple as putting your palm on the reel. but the rod is what makes the real difference in handling a fish or getting your line where you want it.
a good rod will also shock absorb on your tippet better which means less lost fish when you make a mistake.
otherwise, besides fly-line/backing/rod/reel all you really need is leaders. some various tippet sizes. youll need swap them for conditions/presentation often. even after each fish if it gets frayed.
youll want some good longnose forceps and some good nippers. look for forceps with a flat section beside the toothed part. so you can quickly/easily pinch down barbs if the water youre on requires it.
a big thing is glasses. not only do they help you see in different water types but, especially when new to fly fishing, theres a huge risk of that hook getting caught on you or anything else really while learning to cast. protect your eyes. even if you already know what youre doing its better safe then sorry.
gloves are good to avoid line burns and stuff, or you can use some hockey/first-aid tape on your fingers. ive had a roll of first aid tape lasting me over 3 years now for less then 10$
cant say the same about the last pair of simms gloves i bought