Many factors make up a good suit. If you want to know how to dress, and dress well, "Gentleman: A Timeless Fashion" is an excellent book to have in your collection. I suggest you read it, because I'm not going to type for a half hour pointing out features of a good custom made suit. I will, however give you some pointers.
The number of buttons is a personal issue, but a good rule of thumb to follow is the taller you are, the more buttons you can get away with. If you are very short, 2 buttons, a basketball player like shaq can get away with wearing 5. Chances are, if most of your suits have 3 buttons, it's because that looks best on you. (being 6'2" and all). buttons should be made of natural materials, usually horn. Button holes should be stitched by hand.
1/2" of shirt cuff should show. Most younger Americans buy thier suits like thier coats, a bit long in the arm. It does not look bad, but it is not proper.
It would be helpful to know what kind of climate you will be in. Someone from California or Florida will wear a much different weight of suit than someone from New York or North Dakota. 12-13 oz. is the typical suit weight. A summer suit will be "Super 100" or lighter. Summer suits are half lined, normal suits should be fully lined. Lining should be made of Bemberg silk (a soft cotton) or genuine silk and sewn in by hand.
If you get a patterned suit, the armholes and lapels should line up with the checks or panes of the rest of the suit as closely as possible. (i.e. a tailor should try to make the front of the jacket at least look all of a piece) There should be no puckering around the armholes where the sleeve meets shoulder.
Pockets should be made of sturdy cloth and sewn with many small stitches.
Ditto for the pockets.
They should be lined down to the knee, edging tape should be used on the hem and should protrude slightly to keep your shoes from wearing them out. The hem should come down to the top edge of the heel of your shoe.
Inner waistband should be lined and should divide at the back for ease of taking in or out. (i.e. in case you get fat). Pleats are a personal choice and I suspect you already know what you like when it comes to them. Ditto for cuffs.
Armed with this knowlege, you have a reasonable chance of getting a well made "bespoke" suit (a fancy word that means "custom made for you" suit). Good Luck!
Advice from the neighborhood butler