Many people have good advice in this thread. Stuff I agree with *strongly*: Go for charcoal grey instead of dark blue if you only have one suit. Go for dark suit, light shirt, dark tie. Wear real shoes, especially with a 3 piece.
For buying: I'd avoid places like Mens Warehouse and JoA Banks. They deal in volume, and are only an OK buy when they have a sale for buy one get two free or some similar silliness. I feel like Brooks Bros is the sweet spot for most things: they have great selection, good prices for the quality, and most importantly sales help and tailors who actually know what they are doing. I've found dept. stores to be hit-and-miss with the tailoring.
The best suits on a budget are going to be done by traveling tailors who will take your measurements, have you pick your fabrics, and then have your clothes made-to-measure in the far east. I will not buy a suit, or a shirt, anymore that isn't made to my measurements. Sure, you can get your suit tailored, but if your shirt has the wrong sleeve length, then it looks like your jacket doesn't fit. I can't stress enough that the tailoring is by far the most important thing in making a suit look good, and nothing will ever be tailored as well as something that is made to measure. Check around in your area, search style and clothing forums, and see if you can find a haberdasher/traveling tailor who deals in these types of things. I think you can just make it in with that budget, but honestly, it's going to be a tight squeeze for anything good.
Additional notes, since you seem to be interested in how to make a suit style your own. Suit style is all about the subtleties. You can see it in some of the pictures you selected. One person tied a full windsor (perfectly symmetrical knot) one is a half windsor, and the other is a four-in-hand (very rough knot and the collar unbuttoned to boot). I will wear open-lace brogues with a 3-piece for everyday work, but if I have a presentation to the board I'll wear the park avenues (closed lace cap toes). Sneakers with a suit screams "I want to project my style, but I don't have any clue what I'm doing", while a strongly contrasting pocket square says "I'm interesting, ask me more". It, very unfortunately, takes time wearing the clothes, and a not insignificant amount of money to acquire them, but it does make a difference.