As long as you use 301's for all of the pages, you shouldn't lose any rankings. It might take a while before you see all of your rankings come back when you change the URL, but long term, you should be good to go.

Just make sure that there aren't a bunch of spammy links pointing to the .com before you buy it. That would definitely set you back.

Be aware that once you add value to the term, the price for the .com could skyrocket.

Better to beg borrow and steal in order to get the right name in the first place.

If you develop net domain the owner of com domain would rise his price. My suggestion is to get another com domain and still if you want to build on net then think you would get the com domain back.

The same thing happened with me, but it was only just XX amount.

The current owner may have so many domains they wouldn't even notice the .NET is getting developed. Hopefully (when you buy) he's selling it where you can buy through Sedo, Afternic, or another marketplace. If that's the case, he wouldn't know it's the .NET owner buying it. Still, to make sure you can secure it at that price you may wanna buy sooner rather than later.

Once you own the .COM setup a 301 (permanent) redirect from your NET to your COM. That will basically pass all the juice/authority you've built up on the .NET over to the .COM...You can usually setup these redirects from directly in your registrar control panel.

If you do go forward with this, do not redirect your site to the .com. It would make much more sense to redirect the .com to the already developed .net domain. You won't lose any authority that way, and anyone that accidentally types the .com domain instead, will be redirected to your domain.