In order of importance:
black tie
tied tie - preferably one which is sized rather than adjustable. Do allow a good while to tie it as though superficially simple it takes me ages. The slightly lopsided look of a real inexpertly tied one is the sign of a real cool dude.
Only a cad would wear an elastic bow tie and then replace it with an untied real one for that louch look late in the evenening!
Single breasted or double according to taste. I have a double breasted and though it's smart I think a single breasted shawl collar is the best for that old fashioned look.
A.proper dinner suit has no flaps on pockets and has piping down the sides of the trousers.
Tidy black shoes. Get a decent pair of plain black oxfords, made in Northampton. Loakes are best bang for your buck and genuinely stonking value.. Crockets slighlty nicer but f-expensive. Plain black rather than patent leather mekes them wearable in any occasion not just evening wear.
A proper evening waistcoat is OK, especially with double breasted but cummerbund is in my view a better thing. A flamboyant non black waistcoat isn't really black tie, especially if paired with a tie that isn't black.
Get some braces - preferably white if you're after the real look. It's more about that louch look with the undone tie later on than actually holding your trousers up after all. Or maybe for when you've gallantly given up your jacket to keep your lady warm for a stroll outside.
Shirt - a nice evening shirt, wing or normal collar is good, but a tidy smart white normal shirt is acceptable if you're £30 short. Either way, cufflinks of course !
Dress studs a nice touch but not essential.
And there you have it; cooler than Daniel Craig as 007!
As.and aside my first dinner suit was an Oxfam 3 piece double breasted. At my first occasion of wearing it I found an invitation relating to its previous outing. The High Wycombe and district grocers' association annual dinner. The stated bill of fayre seemed rather meagre given the profession of the dinners. The read the first toast "The King". Presumably it had had been worn at least 35 years previously to my purchase. Sadly my ex-wife made me get rid of it. I still believe a slightly ill-fitting vintage suit has a certain something but hey ho.
One thing I would add is that once you have one, you will find excuses to wear it ao £250 or £300 quid isn't quite as daft as it seems. The annual / bi-annual black tie do can easily become the 4 times a year black tie will do and you'll be glad you got it.
or something out on that post 
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