I see that there are some places selling preban
lower receivers, but just lower receivers,
I know the lower receiver is considered (The
Weapon) by law. My question is, If I buy one of
these lower receivers can I purchase a pre ban
style upper for it? and would it be a legal to
own pre-ban weapon ? or is it worth all of the
trouble ?

This question gets asked and answered repeatedly and always returns the same answers:
"yes" and/or "no".  

I think it depends on your risk tolerance- you'll have to decide for yourself.  Lots of people do it, and chances or getting caught are small, but the consequences can be enormous.

FYI, a lower receiver that was never assembled into a pre-ban configuration before 9/94 is NOT a pre-ban, and never will be.  I personally would never feel comfortable with one unless it was configured as a post-ban.  But that's me.