Building a new website on another domain while main stays active - how to code & move

So my main website is ohiocaabis.com and it's on the Invision Power Services platform & I am redesigning a completely new one on the wordpress platform.

However, I do not want any downtime while I'm building and designing the other one, so i thought I would build the new site entirely on a separate domain.

It's already setup & live on ohiomarijuana.com

My question is, when were building links, photos, etc we can try to manually set photos and links & other stuff to ohiocaabis.com, but isn't wordpress going to write a lot of stuff as ohiomarijuana.com?

So how do we change all of that info easily when we do the data base transfer?

Also, what happens to all of the links and stuff out there now for all of my current ohiocaabis.com related material, it's basically going to be worthless and I would lose all of my google rankings and everything won't I? It will be like starting from scratch again won't it?

Second of all, if I am doing this correctly and I need to do a database transfer when finished - this is not an easy task is it? I'm pretty good when it comes to back end hosting stuff, but have never done this before. I know to make backups before hand. My hosting is also with hostgator, any idea if they'll do something like this for free? If not is there a trusted member I could hire for a reasonable fee to do this database move for me?