RAM, CPU & Cooling will be your main needs.
A desktop will be ideal due to you having a huge selection of motherboards.
If you need to go with a laptop you are going to find that having a laptop that supports 16GB+ of RAM will be very expensive as Laptop RAM is a totally different size compared to a desktop.
But please remember this, RAM is extremely important if you are going to be working with samples. Here is my priorities:
1: RAM - 16GB+ (32/64GB preferred) 1600MHz with low timings.
2: SSD - Not HDD, if you are using samples get a Solid State Drive with good READ times.
3: CPU - 4 or 6 Core i7 with hyper-threading will see you well, these will give you 8-12 virtual cores.
4: Cooling - If you are using a laptop then you need to keep it cool while note generating too much noise. I suggest a riser stand with fans powered by the laptops USBs.