They sell premade Wills (last will and testament). once filled out/signed, are these valid and legal Wills?

the signature must be witness by two people, and notarized. then file it in a safe place, preferrably a copy should be left with an attorney. then yes, it is legal.

I believe it must be signed in front of a Notary.Then yes

Yes.

I believe they need to be signed by witnesses and in front of a notary who also needs to apply his seal.

The contents of Wills are not generally the issue. Wills get challeneged because there is allegedly something wrong with the witnessing procedure. You can buy a premade Will, but whether or not you know what you're doing on execution is still a crapshoot.

Look, get real. If you don't have a large enough fortune to pay a lawyer 300 bucks to do it right, you don't have a large enough fortune to care what happens to it when you die.

The laws vary from state tot state on how to properly set up a will. What might be legal in one state might be inadequate in another state. When you purchase "premade" wills, a change in the law might have some impact on the validity of the document.

I once purchased some software the creates wills just to test it out. It was put out by a major company and it had a major flaw that could have cost thousands of dollars to fix. I pointed out the error to the company and they issued and update.

The point is that you should really see a qualified attorney. You will spend more money, but you will receive personal attention and be a lot better off.