I'm much like you in that I don't play gigs and just do my own self projects. That being said I have had the SP4 i5 8GB 256GB since it was released and it has worked just fine for everything I do. I'm using the SP4 with Ableton Live 9, Push 2, and a Novation Launch Control but I also use Reason as well. I have an M-audio Axiom Pro 61 controller but it doesn't get as much use since I've had Push 2. I did get the Surface Hub for more USB ports but I have run my Focusrite 2i4 gen 2 and Push off the Single USB 3 port with a cheap USB hub. I would recommend if you use a USB hub make sure it's a powered one if you have more devices to plug in!
I've never tried Live on my SP4 with the internal audio so I can't really comment on that. I've always used ASIO soundcards for the faster latency. I'm using the most up to date stock audio drivers. I've used ASIO4All in the past but don't have a need to use it now.
The type cover is actually pleasant to type on and gets the job done. I can't see using the SP4 without it. I don't use touch at all for my DAW so I can't even begin to comment there. The screen is nice and sharp and the size doesn't bother me at all. If I need more screen I have two desktop monitors for that.
I think the physical size and weight are excellent. When I travel it doesn't take up much space at all and I usually throw it in a small cross body backup. Most people don't even know I'm carrying around a computer with all my accessories in it.
As for the battery expect about 4-6 hours on average. I haven't done a lot of audio work with just the battery but the few times I did I worked for over 2 hours and still had over 50% battery left. I have used the pen to sketch stuff while sitting at Starbucks for over 3 hours and still have plenty of juice left.
Over all the SP4 is a great buy for the portable and non portable musician. Is it the best? No. Is it the worst? No. All computers have their little quirks that you just learn to work with but I am 100% satisfied with my setup and what it allows me to do. I think the only thing I wish I had done was get a bigger SSD (512GB).