When it comes to selecting a laptop for voiceover work, the requirements are not really that high. It depends on the software you use.

I say laptop, as I'd recommend against buying a 'tower / desktop' PC, as they are large and noisy and you are trying to get rid of any or all noise possible (see section on Studio treatment). Try to get a laptop with 'passive cooling meaning that the laptop cools using it's aluminium surface area, without using moving parts which leads to very low fan noise.

Firstly ask yourself: "Am a 'Mac' person, or Windows person?"
In my opinion You definitely get more bang for your buck with a Windows PC, the specs are higher and there is a broader range of software available, but if you like things easy to use or have multiple existing Mac devices in the mac ecosystem, it may work better for you to work within that ecosystem. 

If you're undecided, look at these articles: https://www.google.co.nz/search?q=windows+vs+mac

For which software is available, see the 'Digital Audio Workstation' section, but I use and recommend Adobe Audition for any and all voice work, which calls for the following computer specs:

Windows

Multicore processor with 64-bit support
Microsoft® Windows® 7 with Service Pack 1 (64 bit), Windows 8.1 (64 bit), or Windows 10 (64 bit)  
4GB of RAM
4GB of available hard-disk space for installation; plus additional free space required during installation (cannot install on removable flash storage devices)
1920x1080 or larger display
Sound card compatible with ASIO protocol, WASAPI, or Microsoft WDM/MME

macOS

Multicore Intel processor with 64-bit support
Mac OS X versions 10.11, 10.12, or 10.13
4GB of RAM
4GB of available hard-disk space for installation; additional free space required during installation (cannot install on a volume that uses a case-sensitive file system or on removable flash storage devices)
1920x1080 or larger display (for best results)

These specs can be achieved with any laptop from the past 10 years, so there's no need to buy the latest and greatest.

Toby in Studio 2018 Web Res (2 of 6).jpg

HOWEVER - the more you spend, the better your computing experience will be, and the better your computing experience, the more stamina you will have for auditions and the more work you can get done. So I'd say spend as much as you are comfortable spending. Any new laptop you buy these days will meet the requirements, but if you want to want to work with video or do things concurrently with your recording software, you may be looking at the mid-to-upper end of the range.

As far as which sized laptop/macbook you get, the only real difference is the screen-size and some slight spec tweaks perhaps. If you like a bigger screen for ergonomics and work efficiency as I do, then I'd suggest buying a lower model PC, and getting a bigger external screen. I have a giant curved widescreen monitor plugged into my laptop in my studio, which lets you work efficiently during editing, but without buying a giant laptop!

I use an Asus Zenbook N501VU, I've had it for over 2 years and it's easily the best laptop I've ever owned!