Besides the basic grooming supplies (Brushes, hoof pick, etc) always keep a first aid kit handy. You can buy pre-made kits or make your own. Just look at the pre-made kits and put similar stuff in there (bandages, bute, wire cutters, scissors, antibiotics, hydrogen peroxide, gauze, cold packs, etc.) There is a kit running $45 in the Country Supply magazine (www.horse.com) They also have a trail first aid kit if you want to spend a lot of time on trails it would be a good idea to have in your saddle bag. I am definately no professional on how to measure for a horse's saddle; however, if you are going to a tack shop looking for a saddle, first take a wire hanger and bend it to the form of your horse's withers, then, bring that hanger in and find a saddle that has that basic shape to it to make sure the withers will not have pressure on them. Saddle pads are a wonderful thing and there are so many different styles/padding/thicknesses that you can make a saddle fit by the pad. I have found that lighter saddles are just as comfortable to sit in yet they put way less weight on your horse's back making them happier. I am not saying heavy saddles are bad at all but they must fit VERY well in order to not make the horse's back sore. Usually the heavier saddles are more for showing, with all leather and silver. The lighter ones are usually half synthetic which make them light and also easy to clean. Country Supply (www.horse.com) has a nice saddle package that comes with everything you'll need to ride for $300-$320 depending on the seat size (depending on the size of your body...have a knowledgeable horse friend help you out with this or go to a local tack shop and sit in a few to see what you like. This is a all-leather, 30 lb saddle that is great for recreational (This saddle is also free shipping!) . Remember the saying is true...you get what you pay for. I have bought a saddle package like this and it is very nice as long as you keep the maintenance up on it (oil/condition it about once a month...maybe not even that often) This saddle package will probably fit an average quarter horse/paint type horse...maybe a little big for an Arab but look at the measurements and measure your horse to see if it'll work...it would fit my Arabian...
When you put the bit in the horse's mouth, make sure it has two wrinkles on the sides of it's mouth. This means the bit is tight but no too tight. Also, make sure you have a chinstrap and when you tighten it you can fit two fingers between the chinstrap and your horse's chin.
I have found Hamilton halters are very nice. They have sizes on them (ex. big, average, small, Arabian...or something along those lines)
Prices for board will depend on specifically where you live. Around here (Southwest MI) it is about $350-$400 for a nice barn with an indoor and consistent feeding schedule and stalling. Look around and investigate. Since this is a pleasure horse a barn with a lot of trails may be more important than one with an indoor arena. Personal preference dictates that one. Also, if the barn you will be boarding at uses blankets, they run from $100-$200 depending on the brand, quality, etc. This will require you to measure your horse from it's withers to it's dock to see what size you'll need. Also, living in northern Illinois it may be a good idea to have a cooler for your horse...especially if you plan on riding them until they are sweaty. The cooler will pull out the moisture from your horse's coat as they cool down helping to prevent rain rot and it keeps drafts off of them.
There are SO many different products and items in the horse world. Some of it will be treasure and some will be junk. Explore a little and find what works for you. DEFINITELY take your time when looking for your first horse and have someone with experience come with you when you see it so they can look for things you may not know about yet (club foot, founder potential, etc) See they horse several times before you put money down and if you can, try to stop out unexpectedly one day. I know it may be rude but you will want to see they horse while it's fresh to make sure it wasn't drugged every time you saw it...I know it sounds crazy but it happens. ALWAYS pull a vet check BEFORE purchasing the horse to make sure it doesn't have anything wrong with it. You will know the right horse when you find it. I looked at three different horses before I found my first horse.
All I can say is have FUN with your horse and just be patient when finding the horse, the facility to board, etc.
Hope this helps!