It can go either way. I think that the avionics package has the greatest chance of getting revised / upgraded over the intervening time so that might best be purchased / go in last. The engine has less of a chance of seeing revisions / upgrades, but sitting there without being able to run doesn't do it any favors. However, I believe that the ROTAX engines are test-run and pickled by the factory so short-term storage / inactivity shouldn't be an issue. Both the engine and avionics packages are relatively easy (and fun!) to install.