I need some advices on expired domains.

I wouldn't paste the old gov content onto your site. You could try to replicate the general theme of the old site. First of all install a plugin that will capture all of your backlinks in. I like to use "404 to start". There is another plugin but it doesn't seem to be updated.

Get your site up with a few posts and add a couple of sidebar plugins or other widgets that will fit with your site's theme.

Don't let it look like it's sitting there ready to take links.

Have a few posts that are just great posts linking out to a couple of authority sites. Get some media (videos and images onto the site).

Never overdo your site in terms of links out to your money sites.

"Otherwise if I generate new content for the expired domain, will the "new" site with the expired domain name benefit at all? "

Yes it will benefit with new content. You don't have to risk copyright or any dodgy content from the previous site. Indeed, it's not really worth doing. The old links pointing towards your site should be good to go, no matter what content you add. That's the way it seems to be at the moment. A few folk believe that that is exactly what will be targetd in the future but it still works as of now.

Keep your eye to the ground and keep up with updates, though.

I know I'm probably in the minority here, but I never use the old site content.

Just post up your new articles with your links and you're good to go!

Works just fine if you ask me, plus avoids the headache of previous owners coming after you and stuff.

Thanks guys for ur wonderful tips.

Say if most of existing backlinks for the expired domain, point to the main page, it will work fine right? So far i checked all the backlinks and they don't point to the inner pages. Thats why i think the domain authority is quite high.

Also, can someone advise me if i am gonna add new content, is it better to have the same topic/subject? Or same niche? Or can be a different niche?

About my opinion you should buy a new domain for your website for solving the problem.