You need to make a decision between laptop and desktop.
Pros for a desktop:
- Higher performance parts
- Cheaper price for same performance
- More expandability, e.g. multiple hard drives and SSDs
- Future upgradability
- Better desk-centered workflow
- Easier capability for multiple displays
Pros for a laptop:
- Portability
I'm a desktop guy. But if portability trumps all those things for you, you have your answer.
Buy or build?
In my experience, it's really tough to beat the price of a cheap desktop or laptop without upgrades. Dell and others have incredible buying power to give you a simple box with Windows at a very low price. Especially if you include refurbs (Dell refurbs are used for under two weeks and factory certified) and sales on refurbs (Dell Outlet has monthly sales with 30% off the already discounted units).
But ... all manufactures charge ridiculous prices for upgrades. Want more RAM? A nicer video card? A second hard drive? A solid state drive? A bigger solid state drive? They charge absurd markups on these.
And it makes sense. Everybody looks for the cheapest PC for the best deal, then they start adding features to get it up to spec. So they have a very low base price but charge a lot for addons and extras.
Building a PC is easier than you'd think. TechReport guides you on choosing parts with their regularly updated guide, and they even have an assembly video guide:
The Tech Report System Guide: December 2015 edition - The Tech Report - Page 1
So ... if you just want basic, probably buy pre-built, especially if you are willing to shop sales and certified refurbs. If you want upgrades, consider installing them yourself. Newegg or Amazon are usually best to buy the parts.
But if you want upgrades and more control over choosing your system, building your own PC is a lot easier than you think. And then you aren't paying twice for parts (RAM you don't want and RAM you do), and you can get the decent enthusiast processors and overclock for even more performance (provided you install good cooling).
What level parts?
This is a difficult question to answer. Parts are always a moving target, prices always change, and not everybody has the same needs.
But here are my rough recommendations:
Cheap audio computer:
- Skylake i3 2-core/4-thread 51-watt processor (or whatever more expensive laptop processor scores equally well on benchmarks)
- 4 GB of RAM (dual-channel)
- 128 GB SSD for Windows, your DAW, and your plugins
- 1 TB HDD for your virtual instruments, sample libraries, and sessions
Medium audio computer:
- Skylake i5 4-core/4-thread 65-watt processor (or whatever more expensive laptop processor scores equally well on benchmarks, if any)
- 8 GB of RAM (dual-channel)
- 256 GB SSD for Windows, your DAW, your plugins, and your virtual instruments
- 2 TB HDD for your sample libraries and sessions
Powerful audio computer:
- Skylake i7 4-core/8-thread processor, preferably the Intel i7-6700K (there is no laptop equivalent)
- 16 GB of RAM (dual-channel)
- 256 GB SSD for Windows and your DAW
- 512 GB SSD for your plugins and virtual instruments
- 2 TB HDD for your sample libraries and sessions
Beastly audio computer:
- Haswell-E 6-core/12-thread i7-5930K or higher (there is no laptop equivalent)
- 32 GB of RAM (quad-channel)
- 512 GB SSD for Windows and your DAW
- 512 GB SSD for plugins and virtual instruments
- 1 TB SSD for your sample libraries and sessions
- 4 TB HDD for internal backup
I hope this helps.