As everyone's exact requirements are different, I'm going to list some guidelines here that will at least narrow the field of devices you should consider:
Basic setup:
* Intel i5 or i7 processor, newest generation that fits your budget.
* At least 8GB RAM (this requirement will increase over time, as it always has)
*
Heavy preference for solid state drives (SSDs) - these do so much to improve overall system performance, including WitPAE; there are many instances where I would pick an SSD over a dedicated graphics card, if I had to choose.
Where you can deviate:
* Your preference for a laptop vs. a tablet and/or a hybrid of the two
* Your preference for touch or non-touch screens
* Size of the device and screen, although you should consider that WitPAE already tends to have small text and wasn't written to resize it to accomodate small and/or high resolution screens.
* With an i5 or i7 processor, the integrated graphics system is actually quite capable - you'd be surprised at the games you'll be able to run; that said, if you want to maximize performance on resource-heavy games (something like Star Citizen when it is eventually released), you'll want a dedicated graphics card for the system. For WitPAE, integrated graphics will be perfectly fine and a dedicated graphics card is overkill if you are using it only for WitPAE.
Some general tips/considerations:
* Always try to test a device, where possible, before buying. Maybe a friend/coworker has something similar. Maybe the manufacturer or retailer has a generous try & return policy. Maybe there are stores with demo units. There are many ways to approach this.
* Research, research, research - besides actually trying them, read reviews, watch videos, ask on forums, etc. to make sure the device will be a good match to your wants. Keep in mind that heat generation and battery life are important considerations in addition to the particulars of the components.
* For non-desktop computers - whether they are laptops, tablets, etc. - assume that you won't be able to upgrade it at all. Yes, you may be able to in specific, limited ways, but there is a lot of customization / miniaturization on these devices that makes major upgrades impractical, if not impossible, in many cases. Don't consider them 'mini desktops' with the same modularity/customization - they are NOT.
* Given their general lack of upgrade-ability, I tend to lean towards buying the largest SSD possible, maximizing the RAM, etc. within the possibilities of my budget.
* To contradict myself a bit, don't go way overboard either - if you know for certain that you are not interested in playing demanding games, rendering video, or doing some other intensive application with the device, don't spend lots of money to buy an expensive dedicated video card just for the sake of having the technology. It will only drain your wallet, generate more heat, and drain the battery that much faster.
* Don't rush yourself - watch Slickdeals.net and Fatwallet for deals. They are good sites because not only do they tell you about particular deals, but those deals are put into a forum format where people can rate and comment on them - so you'll know if it is an exceptional deal or not.
* If you intend to play WitPAE on it, make sure the device actually runs the full blown version of Windows and is NOT Android based, Windows RT, etc.
Just my personal preference - if I had to buy a 'mobile' computer today, I would be taking a hard look at the Microsoft Surface Pro series for a touch-capable tablet device and probably ASUS for laptops. I've had good luck with DELL laptops in the past and would probably consider them too.
Here's a basic demonstration of what I look for when scanning device advertisements - these happen to be DELL but it is the same process for any manufacturer:
So, scan advertisements for basic requirements, get a collection of possible candidates, then start the process of looking at each closely. Apply more criteria to narrow the field until you arrive at the best fit for you.
