You need staples. By that, you need some shirts that actually fit you, some nice ties, a few decent suits, and a couple of pairs of shoes. The most important is shirts and pants if you're building a wardrobe.
Shirts:
Shirts are your bread and butter. I'm a 42 chest and have a similar problem. the remedy is to go out and try shirts from several different brands, and find the ones that are cut to suit you. Also, you need to make friends with a great tailor, who can measure you up and get you sorted.
Pants:
I thought pants were easy, but you would be amazed to know the variations. Most importantly, the number and type of front pleats, the material and how it drops, and the cut play a huge part in how a pair of pants sit on you.
Jackets:
Not sure of the type of work, but you might not need to wear matching jackets/pants. A nice business jacket/pant combo should set you apart with a little tailoring. I find business suit jackets are really boxy, and can always use some cinching at the waist.
Ties/tie bars/ cufflinks/ accessories
Assuming a nice watch is sorted (:op), accessories play a big role in looking good. There was a great article with the wardrobe designer of Mad Men on askmen.com recently, and he put together the clothing for the characters. He discussed thinner ties, presidential cut pocket squares and so on, you could use much of that.
Tailoring:
Most important is tailoring. I would spend 3-4 grand on clothing and then using the rest to get all of your clothing tailored perfectly to your body type and style of wear. Either that, or you spend a lot of time hunting around in shops trying to find cuts off the rack. It's possible. There is an Australian designer whose cuts are a relaxed European cut and not really slim-fit (I have no idea what body-type fits on those, by the way). Tried on a 44 (my actual size and it fit well enough, but was a little baggy. Tried on the 42, and the waist was cinched perfectly to my shape, sleeves were exactly where I would have had them tailored, and the length was exactly where I wanted it. Needless to say, I got both the black and blue jackets
Great tailoring never goes out of style. I've moved from very fashionable to more classic, well-selected pieces. The streams of compliments tell me the changes are for the better
Most important, enjoy it clothing should be fun and rewarding, and not a chore. Look for the fun in everything, makes life more fun to live and if you need inspiration, watch any of the last 5 Bond movies, especially the recent ones with Craig... Tom Ford designed all the clothing and Holy Crap, he's a fantastic designer.
Have fun mate!
SR