If you're going aftermarket, you'll need a CANBUS connector to get the steering wheel controls working, although just what you'll need will depend on what head unit you're looking at fitting; a Kenwood connector won't work on an Alpine and so on. You may actually find that *some* head units come with a connector or support CANBUS out of the box, but they're going to be in a minority. You may also find that even with the right connector for your head unit, some of the buttons still won't work; that's down to the head unit again.
Wiring harness will be the same and will depend on the head unit you end up with, but you'll most likely need something that converts your head unit's ISO block to the Seat's factory wiring; Connects2 do these on eBay for about a tenner.
Again, the fascia surround you'll need will depend on the head unit; there's a theme emerging here! Two types of surround are available; one for head units with square edges (most aftermarket head units are this sort) and another for OEM head units or aftermarket head units that are designed to look stock (Zenec are the most common of these). Once you've decided on a head unit, you'll be able to order the relevant surround in the correct colour from your dealer; Sere Motors do mail order and I, along with a lot of others on here, would recommend them for price and service. The proper Seat surround also requires about a dozen small metal clips to hold it in place; again, your dealer can advise. Aftermarket surrounds are available (Autoleads do them for one) but they're not really any cheaper and I'd stick with the Seat one for a decent fit and colour match.
Only other thing would possibly be an aerial adaptor but, you've guessed it, that depends on the head unit!
If you're going with a genuine VAG head unit rather than an aftermarket one, many will just plug and play, including the steering wheel controls, but some will require tinkering with the CANBUS; I'm not too familiar with that bit though, so somebody else will be indefinitely better placed than I to comment!
R.
