how to avoid broken links

I need some advice on how to set something up. I hope this doesn't sound to complicated.

I am converting an online website publication to wordpress. The old site needs to stay up while I am doing this so I can't use the main domain to create the wordpress site but I do want to use that main url when the site is done. I am trying to figure out the best way to do this keeping in mind I am importing about 16,000 articles from the old one via a file CSV and I am trying to avoid broken links. What is the best way to set up a wordpress site while keeping the old website up so that I don't have to go and make changes to the tables?

These seem to be my only options. 1-is using another domain (like the .net version of the main website) and then doing a 301 redirect on the main domain when I am ready to switch it over to the new site however the primary domain would be the .net and I would like to keep the primary the .com as it has been for years 2-Create an A-record like blog.mainurl.com . I can't remember what the next steps would be once I am ready to do the switch but from what I understand, there is something I can do where wordpress will automatically recognize the mainurl.com without the blog. in front of it and I wouldn't have to go and edit any config files or tables. Maybe even something in conjunction with permalinks? This has to be a common issue for those creating wordpress sites while keeping their old sites up. Can someone explain the process or options to me?

My main concern is broken links. There are all those stories out there for the last 10 years. I want to make sure that if someone has those saved somewhere that they will at least resolve to the main website if not the actual story. There is no way for me to add code to each page because there is too many. (or maybe there is a way to import into the html of all pages a 301 redirect?)

I am putting it on an unlimited hosting account with a dedicated IP. Wont that make anything someone tries to access with the main URL at least go to the main website even if the page doesn't exist,as if it were on a dedicated server? I would rather it go to the main website then nowhere.

Thanks!
dani