Windows 7 OEM version Dell Branded?

Well, not sure if Dell is any different, but from my experience, you can use any OEM version at any kind of PC. As long as the standard policy (one serial per PC) is fulfilled. Aka you stop using the PC you got the serial from.

I got several OEM versions over the years from several brands (like acer, samsung, hp) and used all of them afterwards on a different, custom PC without issues. Just make sure the site you want to purchase them from is legit.

I saw a website (legit, BBB accredited A+) that sells Dell branded, 100% guaranteed and genuine, with COA and everything. The discs is original they said, but are made in China. Are there any differences or restrictions regarding to the discs? I heard that some store sells everything genuine (COA+KEY) but discs are copies. Made in China original discs, and copy discs, is there any problem with these?

Well, you can copy an install disc and it will work just fine. Even though you aren't supposed to do that, officially.

What really matters is a legit serial, cause that is what will be checked when you want to activate Windows. I doubt they can or do check if the install DVD is a copy or not ...

IIRC the Dell branded versions need to have a Dell Bios on the MOBO in order to auto activate - If you do not have a Dell based MOBO the OS will not auto activate and you will have to call MS to get it activated (as long as they are including a legit product code key it should activate - ie. on a Dell system there will be 2 different codes one that auto activates once the Dell Bios is found on the MOBO that is already placed on the recovery disk that comes with the system and 1 that is on the sticker placed on the case for use if you ever reload the OS without using the restore disk\partition )

Whether MS actually allows the resale of Dell branded disks is questionable and probably depends on what you tell the activation rep when you call to activate it (ie. not sure if Dell's license remains valid once resold or if it is only for prebuilt systems) - So I'd pass on the purchase unless a call to MS can verify that it is useable !!

Yeah, better safe than sorry.  

More precisely, you need a Dell brand mobo with SLIC 2.1 table in the bios. It won't work on older Dell mobo with SLIC 2.0 or 1.0.

Your heard right. Except they're not exactly 'copies'. A better term is pirated or counterfeit software.
The problem would be that the supplied license key is invalid and wont complete the Windows Activation procedure.

Any website can say it's legit. And any site can post a claim it's BBB accredited A+.

Did you check to see that, in fact, it was registered with the BBB?
http://www.bbb.org/us/Business-Search/

yes, i checked. i know disks copies are counterfeit, but COA is legitimate, and some are just made in China disks, but legitimate and original.