Yes you do need the OEM version as that is what Dell uses.
As to downloading it, while it would be ok, legally, as far as
the license, I would be wary of possible problems (virus / trojan)
that might be in the download. It would have to be direct form MS
to be safe. Better to borrow a copy of the correct version / type
and make a copy of it. Or see if Dell can provide just the OS on
a cd / dvd (even if it costs a few $$)