Skylake I5 or I7

 I may be wrong but an i7 has hyperthreading and the i5s don't.

Soundcard wise I already have a Focusrite 2i2 and Im fairly happy with it..... still getting used to to using an external card after so may years of Soundblaster audigy & M audio 24/96  internal interfaces

Lots of people do run DAWs on i5s, especially in the Mac world (lots of Apple computers have i5s as standard, with i7s an option on some) and Windows laptops.

Having said that, a DAW is very much a "specialist" use for a computer, and makes similar demands on the hardware to a gaming machine in every respect but video and makes harder demands than gaming on the ability for near real-time audio. "Normal" computer use, including gaming, doesn't require the computer and OS to be giving constant attention to audio streaming with no gaps longer than whatever the interface audio buffer is set at. If Windows looks away from Word, a browser or even a game for 15 or 20 milliseconds it's unlikely the user will even notice. A DAW running at low ASIO buffer settings to get low audio latency will notice and audio will crackle and pop or drop out as soon as the ASIO buffer is empty. So if you have your ASIO buffer set to give e.g. a 10ms round trip, as soon as Windows looks away to meet another system request for more than 10ms you get audio engine dropouts. Processor speed and power helps keep Windows' attention where it is needed, as do more cores.

If the budget runs to it I would go for an i7 over an i5 every time. With perhaps three exceptions. An affordable i5 with a significantly higher clock speed than the fastest affordable i7 may have an edge in some ways, so long as the number and nature of plugins used doesn't mean the i7's hyperthreading (or extra cores in the case of mobile processors) would make a difference. The second exception is if you're using a laptop and extended battery life is important, as i5s generally demand less power than i7s.

The third exceptin is that if using an i5 rather than i7 means you can't get low enough round-trip latency figures and/or need to freeze tracks more than you would with an i7. Only trial and error can really confirm whether that would be an important issue for you. Freezing tracks just takes a little time (though it irritates some people because they find it a break in smooth workflow), but an inability to get low enough latency can be a real show-stopper.

Another relevant issue is that poorly written ASIO drivers can substantially reduce the performance of the entire DAW, so in many ways it's best to consider the proposed system as a whole. If the alternative to an i5 and a good interface woth good drivers is an i7 but a lower quality interface with indifferent drivers I'd go for the i5 in that situation.

USB3 has more bandwidth than USB2, but unless you're recording a serious number of inputs, way above the 16 or so most larger interfaces have, it won't make any improvement over USB2 because USB2 has more than ample bandwidth to cope. I can run my UFX/ADAT combination over Firewire 400 without maxing out the bandwidth.

Where TB may have an advantage is in performance for lots of inputs, such as the kind of setups found on high-end PCIe based studio systems with big desks. Even they don't beat the best USB/Firewire interfaces by more than a very few milliseconds though. Even optimised digital mixers (or digital guitar fx) add some latency.

The difference between 5 milliseconds latency and 2.5 or 3 isnt really very meaningful in real world usage, and any plugins used will probably add enough latency of their own to even out the performance.

The big thing about TB, to my mind, is that it allows PCIe speed hardware to be added to laptops (and it's the only way to add PCIe cards to anything Mac). A laptop with TB and a TB hub gains quite a lot of scope for expansion, even if it is wired. Which is useful to people who spend some time working in a studio-type environment but also need a portable computer.

As Jim says, the catch with TB is that anything TB is currently at premium prices. A basic single 2.5" TB2 drive enclosure costs more without the drive than a complete 2TB USB3 drive. Another issue is that much of what's marketed as TB/USB3 is really USB3 internals with a TB socket welded on. Which means you can't use SSDs in such a case and run the TRIM command on them (TRIM commands can be passed over TB but not USB). It's like the USB drives marketed as "USB2 and 3 compatible" or similar, which means USB2 but will work if connected to a USB3 socket.

Rather how the TV industry markets 720p TVs as "HDTV Ready" in the UK, a weasel-worded practice especially common shortly before HDTV was launched. What they really mean is "not actually, real, proper HDTV but it can decode the signals.” Or how a Gigabyte is 1024 Megabytes unless you're a hard drive marketing manager when a GB=1,000MB. :-/

i look at it even more simply than that...
 
least hamstrung for future upgrades.
 

I went with a cheap Dell i7 desktop with 16GB ram from Amazon.  It is a huge improvement over my previous quad core AMD.  I also have an i5 laptop with 8GB ram and the i7 runs substantially better.

edited.

"The lastest Macs are a great example:
The current top-tier Mac Pro runs a Xeon CPU (hex core).
The current top-tier iMac runs a Skylake 6700k (quad-core).
This iMac is *faster* than the current top-tier Mac Pro.
Faster clock-speed and more recent architecture... "
 
The whole iMarketing here seems a bit odd. :)

Apologies for the duplicate posts. 
 
I started to wonder where this was all headed back when Apple adopted the same Intel chips as PC. 
Some Mac buyers really have no interest in seeing the numbers or knowing about the "innards". Marketing has taken the thinking out of the equation. " I want the best Mac, is that Mac Pro? I'll take it".
 
Value perception is another thing I've noticed. Some people will buy a 5 year old mac instead of a new PC. I have no problem with any of that. It means that the computers work and marketing was genius. If I were a Mac user, I believe I would at least look under the hood and compare. 
 
And I don't think people care as much if their workplace  is buying the computer for them since it isn't their money and they know that the computer will do what they want it to do.