That's going to be very difficult to find used and impossible new. Steam lists a quad-core CPU, 6GBs of RAM, and a GTX 760 card as minimum requirements.
What you need for The Isle is something like this $750 MSI:
https/www.newegg.com/Product/Product.aspx?Item=N82E16834154149
MSI laptops generally offer good quality and decent specs. This model is an entry-level machine with enough power to play The Isle without problems.
But if you're OK with a used laptop, then this $200 Lenovo Y50 found on eBay is worth looking at:
https/www.ebay.com/itm/Lenovo-Y50-70-15-6in-1TB-SHDD-Intel-Core-i7-4720HQ-2-6GHZ-8GB-RAM-GTX-860M/163023006160?hash=item25f4eda9d0:g:jOQAAOSwx8pa5LjZ
This is a fairly old model by now, but I have one myself with almost identical specs including the quad-core i7 CPU, the 4GB GTX 860M GPU, and the 16GBs of RAM. The pictures are very low quality, so it's difficult to determine the exact condition of this machine, but if I were you I'd get the Y50 now and start saving up for something like the MSI mentioned earlier.
Cheers,
GreyCatz.
PS: The eBay seller is very new (March 2018), so my advice is to contact the seller and ask about the general condition of the Y50. And it's also an auction, so the price is likely to go up over the next six days.