Another 2 cents worth:  Buy a laptop that is optimized for audio.  I got one from ADK computers (adkproaudio.com), but there are others.

USB 3 isn't entirely reliable for USB 2 audio devices. It would be wise to have at least one USB 2 port.

Other than that, the discussion is focussed a lot on latency and from what I gathered before, you don't care about that. Which means you'll be able to get away with a LOT more. Add to that that you will be basically using one piano based off samples on an SSD and I really don't think you need anything particularly top of the line, let alone an expensive audio interface. That, of course, remains an option for later. But anything i5 or better should be ok.

Of course, if you're planning to expand your use of the laptop later, of course it makes sense to dig in and get an optimized-for-audio machine.

Sanderxpander comments hit the nail for me: basically I have a beast pc at home which has been completely reliable for live playing.  But when I'm on holiday or just out of town, I'd like to be able to edit music.  Not to play it in, just to listen to stuff I've already done via Sonar Kontakt and various sampled pianos, and tweek it.  So I just need an accurate reproduction of what I've done, so I can mess with the audio or midi file, etc.
 
This issue has to have been raised before.  I should do a more thorough search, may be.
 
 

I've just upgraded from HP Envy and although a lot of people complain about HP, it did the job, was reasonable and I never had any compatibility issues.  I'm now experimenting with a Toshiba Satellite (I know) which is a desktop replacement, 16 gig of ram 4th gen Haswell i7...so far and touchwood it seems to be a beast.
 
It's future proof too, it can beam out the monitor to a DLNA networked TV (I think this is how it works)...haven't used it but I've used mirror cast before.  It can hook into a 4K monitor via the HDMI port, it can also hook into a 3d TV via HDMI port and show 3D movies.  The screen is a touchscreen too, although it isn't the best LCD panel...I'm learning the difference between IPS and TL (I think) at the moment but it's no biggy because of the extension options.  IPS is only really for hard core video, photography and gaming...although I will get one of these panels in the future.  Oh it's HD 1080 and all that jazz.
 
I'm experimenting with an external USB 3 SSD drive as my recording drive too, and this is interesting.  I haven't tested it with hard-core video editing yet, but I ran an old project last night only 18 tracks but very cut-up...lots of edits and all that jazz, and it swam...I transferred two gig onto it in under a minute and a half, I'm going to run some more tests over the coming days but I am very confident that we have entered a new era in regards to hard drives.  For the record I'm using a Samsung SSD 850-128gig Pro, with a Vantec Nexstar case which says it's specifically designed for SSD's and 6GB SATA III.  Because it did not come with a specific cable, I could not use the magician software which came with hard drive-which apparently would boost the speed even higher but when I ran a speed test...it didn't matter, the read and write speed was about 470MB per sec and the drive is guaranteed for 10 years. Apparently it can handle 40 gig's of transfer a day for 10 years.
http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/html/ssd850pro/overview.html
 
Ben

If ultimate mobility is attractive to you, a guy I know is very happy with a Microsoft Surface Pro 3.

 
Thanks.   These are really great suggestions.  I've got a little Scarlet red box lying around.  I'll probably save a little by mating that to one of the above.   
 
Are chipsets an issue?    Anything I should avoid?

Here's a thread with some people who are using a Surface Pro 2 and 3;
forum.cakewalk.com/m/tm.aspx?m=3169400

As for chipsets, I think an important thing is having a dedicated USB2 port as USB3 sometimes gives problems.

Oh and try to stay away from stuff that promises extreme power saving.

I wasn't talking about Round trip latency, Agreed that is not an issue unless your using the laptop for guitar effects or synths live. I was talking DPCL latency which if you don't already know what that is you might need to look into it. 
 
DPCL is what causes CPU processing spikes that will interfere with your DAW's performance. Things like Battery Management and WiFi as examples of background processes that are notorious culprits. But there are many other processes and some cannot be shut down on certain models of laptops because of the bios. This is why it can be a 10 to 1 crap shoot. 
 
My son bought a $1,500 Dell laptop for audio. It was a dud. He even tried to get his money back but Dell said it is not broken. DPCL spikes every 30 second way up there in the red means nothing to computer geeks. 
 
So he went down to Wal Mart and bought a $500 Acer which he has been using for 5 years now. He upgraded the Hard Drive and the RAM. 
 
My Laptop is a 2004 Toshiba P4 1.5 Gigs of RAM. 250 gig 7200 RPM HD. Xp 32bit.  I can record a whole band with it into Sonar 8.5  running my Tascam us1641 in asio mode. In MME mode I use the on board sound and I can edit and mess around with midi files. Only the TTS-1 is stable however. Dim Pro will kill it. Worst is screen refresh is deadly slow. 
Not trusting the above machine for a paid recording session, one day I borrowed my wife's office Lenovo T450. i5, 4 gigs RAM, 7200 RPM drive.  I was not allowed to mess with it. I installed X3 and using the Tascam again it worked flawlessly recording 8 tracks of a live choir performance. Without optimizing it had DPCL under 75 ms flat and steady. I'm waiting for her boss to upgrade her and I want that one! 
 
So bottom line, don't worry about the brand or CPU. Worry about the bios and DPCL first. 
Get a 240Gig SSD  OS drive. or at least a 7200 RPM. that's paramount for audio. Why external? that's messy? 
A lot of people pull the DVD drive and add an extra 500 Gig or more 7200 drive. 
You can put the DVD drive in an  external encloser for the rare time they are needed these days. 
Get at least 8 Gigs of RAM 
 
I use all USB 3.0 for my studio gear without problems.  Scarlett interfaces are verified to work with USB 3.0
 I only use USB 2 form my mouse. 
 

Dell is out... I figured as much.  

Good point, CM. I've never had this happen with any of my laptops, but I do always remove any bloatware and manually manage power profiles and drivers.

I would consider that Dell story completely unacceptable by the way. A spike every 30 seconds is significant not just for audio production but also video stuff and gaming. Basically all it doesn't matter for is office work and browsing.