I've had quite a bit of experience with audio interfaces over my time.
Now, when you first start, it's going to be a price point situation, and it's trying to get your best bang-for-buck.
Also, keep in mind that when you're starting off your ears aren't tuned, your hearing palate hasn't developed. So even having the best converters aren't going to do you much good, because you don't know what sounds good and what sounds bad.
A lot of people don't have a huge budget when they start, so they kind of look around and look at what everyone else has. Now, I would suggest spending at least about £1000-£1500 on an audio interface to start.
The way to decide what kind you need is basically down to how you're working: if you're a recording engineer, you'll want a lot of inputs and outputs, so that you can record in. You want some pre-amps in there. If you're just a mix engineer, however, and you just want a 2 bus going out, then that also makes a big difference. You're able to get a lot more bang for your buck if it's just a two-in, two-out device.
If you need more outputs, then you're going to need a lot more A to D's and D's to A's, so the quality is going to go down. What you should do is keep it in the box and just have two outputs when you're starting, something like a Prism.
There are several brands to look out for, like Prism, Crane Songs, and Lynx. I've got a Lynx Hilo here, which I've just changed from the Prism. But that's more down to a taste thing and down to the inputs and outputs that I want.
Really, it's a balancing act between the ins and outs you want, how they sound to you, and your budget, so you need to work all of that out. But I suggest you spend as much as you can because it's a fundamental part of your equipment. If you can't hear your equipment correctly or get them as clean and as neutral as possible, then you won't know how to get the sounds that you want to create.
So keep the budget as high as you can, and don't buy anything else apart from a good computer, good converters, good speakers and a good sounding room, so they all work in combination with each other.