You can get a used iMac "really cheap" (same price as new laptop), it's worth consideration if you enjoy trouble free computing and easy as iPhone interface.
However: have a goal of "late 2012 or newer for imac" (for notebook it differes - google it), becuase Mojave update is coming soon.
Your main concerns should consider:
Mathematica supports "OSes" not specific hardware. For Linux this can be very tricky. For Apple High Sierra and Microsoft Windows 10 it's easy. The hardware inside matters: that cheap notebook may have 3D graphics that work poorly, might not. You have to get really specific about features and DRIVERS to know that.
(0) if your a student: carefully check your "class requirements" for what computer they may demand. for example: you may be required to have nVidia or equivalent graphics to run AutoCad, you may be required to have Windows 10 for a computer Cisco class (even tho cisco isn't win10)
(1) Apples are quality and have BSD, which is linux freeBSD friendly. driver updates are released by Apple. iPhone apps can be made with little effort and you can see them run on the iphone while debugging. apple tends to respect "privacy" more, to today's date anyway.
(2) Microsoft PC's run on china that requires drivers that don't get updated (if you buy quality hardware you get updates - but your talking about a cheap laptop?). Windows 10 now is Ubuntu friendly. Microsoft is andriod developer friendly. Win10 is better for gaming for various non-technical historic reasons.
Apple iMac have a better keyboard, mouse, and easier to use interface - tho you may be able to hack Windows 10 to use "similar" look-alike devices. It's easier to do homework when you are not fighting interface quirks or a "hard to use keyboard".
4GB will work but 8GB is wise for upgrading reasons. today's new OSes burn though more than 4G in normal use.
video card memory can be a stopper for games: there seems to be a 1 Gigabyte minimum developing so "roaming" areas are not too small. Intel's "in cpu" directX 11.x allows up to 1.5G which it takes out of memory (another reason 8G may be needed)
used imac (not notebook): $400 and up
gaming PC / windows 10: $300 and up (with paddle)
surface pro roughly 2017 "Core m3 and Core i5: Intel HD graphics 620
Core i7: Intel Iris Plus Graphics 640"
it makes ALL THE DIFFERENCE which icore is in it and which graphics are on board. that cheap (in your local store) surface pro might be a 2016 the store is trying to get rid of, might not.
if you want "smooth 3D modeling" in mathematica to look and work right, i'm afriad your going to have to get more particular about the actual models your looking at
