Just go to Walmart or Best Buy or your local equivalent and buy a cheap laptop, something that’s $600 tops and feels like the plastic is just barely held together.
That would get you a reasonably fast processor and probably at least 8GB of RAM. That's not as low-end as many of your customers may be using.
$600 would be a prime dev laptop for me. I use the cheapest, non-chromebook, I can find then throw FreeBSD or Linux on it. I prefer desktops and spend much more money when I build one of those.