Microsoft has a student program. Check the microsoft store. They will want a college email with .edu ending or something that proves your really a student and you might also try Dell.
https://outlet.euro.dell.com/Online/InventorySearch.aspx?brandid=6&c=uk&cs=ukdfb1&l=en&s=dfb&frid=152&AspxAutoDetectCookieSupport=1
you might call them
autocad doesnt utilize multi cores much so 4 cores will work good. Get a cpu with fast single core speed. CAD likes memory so if you can swing 32gb, you will be able to work with larger cad files more efficiently.
Check Dell Precision refurbished workstations or they have a good end of year sale. They are built for autocad. When I was using them I made sure to get ECC memory. Refurbished units do come with a warranty and Ive had great luck with them...
Try a larger SD card and get a cheap SD card reader (usb interface) that you can plug in to your computer. Then you should be able to manage the files better.
Dell has some good prices now for 4th of July Sale. This one might be too big but it would be a good one. You might check on their website. Look for an 8th Generation CPU since they draw less on the battery and should last longer while not being plugged in...