i5 or i7 with 4 cores (is enough); 8 to 24 GB RAM; mid-range graphics card.
Any suitable mobo will support 5 or more SATA drives and probably 2 SATA2 drives.
If you want an external drive, look for USB3.0 ports - much, much faster than USB2.0.
Switching between applications is likely to be smoother with more cores but acad mainly uses only one and, occasionally, 2 cores.
In fact, it uses only 1 or 2 threads and many of the newer processors claim to run 2 threads per core.