Most kits from GP usually are built around one certain engine and tank. If you are going to use the same engine and tank you'll have no problems moving on now. But if you are planning on using a different engine or tank you might want to hold up until you have them in hand. I say this because there may be modifications to the fuselage, firewall, or cowl to accommodate the different equipment and it's a lot easier to make changes now than it is later on in the build.

Hope this helps

Ken