So you have to shop shop shop.
The Windows 10 issue will get solved so I would not dismiss a W10 laptop since you are stuck on price here.
That said I'd like to see the usual Intel i5 and 8GB RAM to start with. I find very few with less than 500GB HDD so that would be my baseline.
Most of the posts I found warnings about Virtualbox and 10 are way outdated. Prior versions of 10 and Virtualbox.
HOWEVER. THERE IS ONE THING ABOUT 10 I MUST SHOUT UP HERE. Ready?
"Use a local login."
Hi I am looking for a $500 USD or under laptop for running virtualBox. This is the recommendation from the forum.
Any suggestion ?
"Based on the forum postings from the gurus, don't get a Windows 10 laptop, if possible: Virtualbox is not running well on Windows 10 as a host OS yet. Windows 8.1 is good.
Other than that, the more memory and physical cores (not hyperthreads, they don't count) you can get for your money the better. Needs to be a regular processor, not an ARM processor. 64-bit is best. Check for being able to use VT-x or AMD-V in the BIOS, so you can run 64-bit guests or multiprocessor guests. Mind the size of the drive - modern OS's take dozens of GB, consider what OSs you'll use as guests and how many guests."
https://forums.virtualbox.org/viewtopic.php?f=6&t=70290

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