I know some people will tell you otherwise... but I've been building / upgrading PC's at home (for myself and friends etc.) for many years now (I even remember paper tape / card punches ... so that will give you a clue how long) and always use OEM versions of Windows and have "never" had a problem. Even when upgrading a PC with a new motherboard. Though big upgrades tend to cause Windows Product activation to kick in. Usually you have to phone (automated system) and get a new code to type in. Only twice have I had to speak to a "real" person and once I explained, I still got a new code.
So I would say "no problem, buy OEM.
PS. Obviously you don't get any phone support should things go pear shaped. But hell, that's what forums like this are for.