Best desktop environment for a Pentium J1900 ITX System?

Web/Email only client for my grandfather that always had problems with dust clogged fans. I imagine the fanless quad core platform with the SATA3 SSD should have manageable desktop performance, but what OS would be the best option? Thinking about a simple arch install, ubuntu, or CentOS. Windows 7-10 would be best but would it be the best hassle free experience?

I can't make a decision, what would you pick?

Memory: G.Skill Ripjaws Series 8GB (2 x 4GB) DDR3-1600 Memory ($52.99 @ Newegg)
Storage: Mushkin ECO2 120GB 2.5" Solid State Drive ($52.99 @ Newegg)
Case: Inwin BQS656.DD120BL Mini ITX Desktop Case w/120W Power Supply ($77.98 @ Newegg)
Optical Drive: Samsung SN-208FB/BEBE DVD/CD Writer ($16.99 @ Newegg)
Motherboard: ASRock Q2900-ITX (Purchased For $103.99)
Total: $304.94

Try Linux Mint with its default desktop and see if that runs fast enough for you, if it doesn't then I'd recommend xfce as it is light, and usable. Mint is a very 'it just works' sort distro, so you shouldn't have any trouble. Just remember to put icons on the desktop, install any printer drivers (if not already working by default), enable the firewall, etc. to get it out of the box completely set up so there wont be any hitches for him

In terms of a hassle free experience, it's hard to say as to begin with I personally haven't had very good experience with most adults and eldery having very good knowledge of how to use a computer so which operating system for basic usage really wont make a difference as they will most likely still have issues.

To install xfce here is the command for future reference:

sudo apt-get install xfce4

Let me explain to you how command line and terminal is not a stretch for old timers. ") Since it started there. So dont treat old people like idiots just because you are use to a gui. Yes, Linux Mint is pretty user friendly.

I have a similar system. J1900 CPU, 4 gig of RAM, SSD. although I have an additional video card in it for triple monitor support. So I cannot speak for pure graphical performance. Windows 8.1 Pro runs just fine on it. If MS is to be believed so will 10. I have Mint running on it at the moment without trouble. I would use a Linux distro though since downloading ad-ware ridden software is less of a problem than in windows (to be exact it is NO problem at all). To make your life as admin easier I would choose an Ubuntu distro since they have a GUI for everything (nearly everything).
Depending on what your grandfather had before I would recommend Mint (for former windows user) or elementary OS for former Mac OS X user.
CPU performance is just right. Firefox with 15-25 tabs open, VLC playing something and some productivity tool running work just fine.