Yes, salt licks can be left outside. Of course they will dissolve in water, but usually tend to last well enough in average rain. Don't know what your 'brown' salt lick is, but I'd ensure you get one specifically for horses, if it has other minerals in it and also avoid ones with grain &/or molasses in. I personally put out straight rock salt & provide their other minerals in a 'ration balancer' supp.
Re rugs, if the horse has access to shelter and has free movement - isn't cooped up in stall or pen - then they don't generally need a rug. Horses are actually evolved to be outside animals without rugs
Heavy rugs can put too much pressure on skin deep muscles and cause atrophy, as can leaving a horse rugged for long periods. IMO if you're going to rug a horse, it should be as light as possible and should only be used on the coldest nights, out of season cold snaps, or after exercise on a cold day for eg, to allow the horse to cool/dry slowly. If you decide for some reason you want to rug your horse over the entire winter for eg, then a lighter rug is more important and you should budget for getting it off him for at least an hour a day and on any days that are warmer, so it doesn't become a 'sauna suit'!
Don't know about the Nutrena, but I'd avoid sweet feed, except as perhaps an occasional small treat - it's basically junk food for horses. Despite many traditional practices & ingredients, horses don't tend to do well on sweet, starchy feeds, so it's best to avoid grains & other high starch feeds. Your horse, if healthy & not in hard work, should be perfectly fine just on hay/pasture, plus whatever supp that is appropriate for his situation, to provide balanced nutrition. feedxl.com is a great resource for dietary/nutritional info. safergrass.org is a good resource for learning how diet(particularly starch/sugar) effects their health. If you are going to 'hard feed', it's important to feed little & often & avoid large &/or infrequent meals. Horses have evolved as 'trickle feeders' and need a tiny but constant amount of feed going through their digestive systems.
Re hay, if it's fresh & green it's likely got more nutrients in it, but green or brown, hay doesn't lose sugar content over time, and is also likely still deficient/imbalanced in many nutrients, so I personally tend to give precedence to low-NSC level hays(generally native, not 'improved') rather than colour. Of course, a nice fresh, green native hay would be best!
What else should you have in your tack area?? Where should I stop?? Disinfectant, manuka honey, bandages & other stuff for first aid & treating infections or wounds. A hoof rasp, and if you're going to have your horse shod, shoe pullers, for those emergencies where you need to pull the shoe pronto. A soaking boot, perhaps fly veil/mask, bucket & sponge, brushes.... oh and zinc cream or such for a horse with pink skin, to prevent sunburn.
Sounds like you have lots of research to do. In addition to links given, I'd get yourself some good horse-keeping books & look at some horse sites online which have info on management. Of course, as you'll read in my signature link post, you'll get many differing opinions on whatever subject, so it's just a case of educating yourself the best you can, learning all the pros & cons of various specifics and coming to your own decisions.
Oh, fly spray - I don't tend to bother with it much, except if I'm training or trimming a horse & the flies are distracting, but when I do I just use Tea Tree oil or Eucalyptus in water or such. Non toxic, non irritating. Doesn't last that long, but don't need it to. Spose it depends what the flies are like in your parts, too!