David,

You can build it first, but plan ahead on a few items that will be hard to get to after you "close it up". The most difficult items to install after completion are all in the closed up front end of the airplane. Notably fuel tank and firewall items such as throttle cable, fuel lines, engine mount, nose gear mounting and steering cable, etc. If you use the engine installation shown on your plans, you can pre-position all those things and they'll be ready for your engine installation later.

Just plan ahead and you should be fine.

Paul