Anything I need to know before doing complete reinstall of PMDG products after new system build?

Nope. If it happens that you've used up all your activations just register at http://support.precisionmanuals.com and open a ticket. They'll reset it if you need them to.

 

It'd be a good idea to go ahead & go to that site now before you start. There's a good kb article on installing fsx that'll save you some headaches later on.