I'm working to transition from lower business casual (khakis and polos) to higher business casual, which will involve more fitted shirts and occasionally suits. I'd like to do it correctly...In addition, nearly everything I get must be tailored, or often built. Long arms and torso, and very little fits me off the rack or even from the package.Matthias, dear reader, it sounds as though you are afflicted with the gangly. I am not a gangly person myself, but some of my best friends are gangly. Many of our nation's most famous power forwards, strong safeties, and presidents have been gangly. You know who else was gangly and dressed like a ****** *** ***** **ing *** ** * boss? Cary Grant. You are in great company.
Your biggest problem will be shirts. You, like every other reader of this blog, will definitely have to order shirts with a specific sleeve length (no more "34/35" for you) and collar circumference. You will have to go somewhere to get measured--your local menswear shop can do this. You will probably never buy a dress shirt in a store again as a result, except perhaps Brooks Brothers or Thomas Pink (drool).
Start out at Paul Fredrick; their shirt skirts are longer than is average these days, which is good for a long torso. Introductory white oxford deal here. Good place to start for you sounds like button cuff with a button collar (assuming you will wear a tie only irregularly). Get 3 with button collars and one Windsor spread for when you wear a tie with a jacket.
Last, if you have a long torso, you will probably need pants with a longer rise. This means you will probably not look proper in modern-cut suits like the kind you would get at Banana Republic or J. Crew. Pants length is not a problem if you buy pants with unfinished hems.
JAB carries a lot of long-size jackets and suits, and their default pant fit has a lot of room in the crotch (easily handled by your alterations tailor, not the one that works in the back at the local JAB). Never pay more than 66% off list price at JAB.
Drop another comment if you have more questions. I live to serve.