Black is fine if you can't find a decent charcoal. Just make sure the charcoal is dark and not a light colour. Light grey shades are ok for weddings but not really formal enough for interviews. Don't go 3 button. They go in and out of fashion, plus you have to have a very specific build to pull one off. Only tall and well built men get away with 3 buttons. Short guys look horrendous in one as it makes them look shorter and skinny guys look like they are a child wearing an adults suit.
Wool for sure, as has been mentioned. I try and buy Super 120s for that particular type. Super 110 is fine if you can't find a decent 120. The thread count isn't fine enough on a Super 100 so I'd avoid it. Don't go over 120 as the suit becomes too frail then and won't last if you wear it too much. Reserve 140 and above for the time that you spend thousands on an epic black or midnight blue suit that you wear once, maybe twice a year and the odd super important business meeting or interview (most people have no need in other words

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DO NOT EVER DO UP THE BOTTOM BUTTON. I repeat... NEVER DO THE BOTTOM BUTTON.
Ahem, sorry. That one really shits me. Also undo your jacket when you sit down unless you're reading the news.
Don't dryclean more than a few times a year. That ruins the fabric and will make a suit last 2, 3 years tops. Get the drycleaner to "press" it instead. If it's stained, then of course you dryclean it and make sure they spot clean it. Also always dryclean/press the top and bottom together, so they stay the same colour.
Go have a look at the Pierre Cardin as SM mentioned. They do pants and jackets separate so you can mix and match. I've got two that I wear for work, excellent value. They also have the natural Euro cut that tapers in at the waist and the sleeves are cut higher. Never get a classic boxy cut, it's the worst look ever. You suit jacket must be comfortable but form fitting, unless you want to look like a poorly dressed American with a suit 3 sizes too big.
If you get a PC from Myer, they are doing some free alterations now too so you can get the pants brought in and perhaps the jacket if you need a more slim fit.
Waistcoat? Nah, don't worry about it. It's a nice look but a good three piece is tough to find off the rack that won't cost you the same as a small home loan.