I'm getting married, and for the ceremony I'm going to buy a suit and have it tailored.
Congratulations! Opting to wear a suit rather than a tuxedo has become a very popular alternative in groom formal wear in the past few years. And I tend to think that there is some wisdom in your choice to do so. For one, a well-fitting suit would be a
much much more versatile addition to most guy's wardrobes as opposed to a tux. Two, not only will many of the available rental options be comparably priced to purchasing an entry level suit, but in general it is nearly impossible to attain a
perfect fit with rental formal wear, which like others have suggested is exactly what you should be striving for.
What are the most important measurements to have fit properly, and which ones can be adjusted by the tailor?
As Dave suggested, that's 'their job' not yours. But in general, it is important to know a few key references when determining proper coat size. In US sizing, the chest measurement in inches refers to coat size. In regards to EU, you generally add 10. So, US 40 = EU 50.
Torso length? Shoulders? Waist? I know the inseam and sleeve lengths are easily adjusted, but other than that I have no idea what to look for.
The length of coat is determined by your height and can vary depending on body type, but generally speaking...
<5'7 = Short, or S
5'8-5'11 = Regular, or R
6'< Long, or L
A skilled tailor can make nearly any adjustment you can imagine to a suit, with the exception of the shoulders. While it is rare to make minor adjustments to the shoulders on a coat (such as adding padding), in general this is the very first component you should be fitting for, as there can be no adjustment to narrow or widen a shoulder with an off the rack suit. One tip to remember, in even the same size, different designers will fit differently so don't be afraid to try stuff on! From the way you described your body type, I'd say that "slim" or "modern" fits are going to be your best bet.
And Men's Warehouse is teh suck!
As a side note, despite Corey's comment, The Men's Wearhouse
does in fact guarantee their tailoring, with only the exception of garments that were not purchased from them. Their tailoring policy is fairly simple, they charge by the seam, so you only pay for what is needed. Once a seam has been touched, it is free to re-do for life, just in case you happen to fluctuate in weight or what not. And it just so happens that many of their tailors come from second, third, and fourth generation families who have made careers from mastering their skill with a needle and thread. So yeah, they're good.
One other suggestion I'd make is to check out this website:
www.indochino.com
They've been around for a few years, and offer a pretty unparalleled service. It's an online only custom formal wear shop that also guarantees their tailoring, plus their prices are very respectable considering their product.
Lastly, a couple of important things to keep in mind would be to figure out your price range, and stick to it. And in regards to time frame, I'd recommend giving yourself at least 2-4 weeks for tailoring just to be on the safe side.
Good luck!