Might want them both soon
I was planning to get the avionics kit, then the power plant, with significant time between them, based on the order of the section in the instructions. what I got was both of then close together and reasonably rapidly after the finish kit was delivered. Actually I got the avionics kit just before the finish Kit and the power plant a month later.
This was actually an advantage because you will end up jumping between them. The finish kit includes the cowling and a note that is should not be installed until the engine is in place. The avionics instructions in some places assume that the engine wiring is already in place, but there is a note in the final assembly instructions (section 38 Finish kit) that indicates that it would be easier to install some of the wiring and the fuel pump assembly before the tail is joined to the fuselage. Working around back-ordered parts also didn't help. (Side note: when you do get close to being blocked by a missing part - CALL VANS. They have been very good about getting me back-ordered parts when I got to a stoppage.)
I was able to do both the fuel pump and the main wire harness before the join. The fuel pump would definitely be a PITA to do laying in the baggage area. Doing the wire harness didn't save as much, simply because the difficulty is getting it into the bottom of the fuselage, which is the same whether or not you have a tail connected.
If I had to guess - the instructions seem to have been written for the avionics to be installed after the power plant was done, even though they have lower section numbers. The avionics sections have you removing a the tunnel and other covers that were installed in the (later) engine sections.
What ever you do, be rigorous about keeping track of what is done and what is to be done.
However that is one mans situation. Your mileage may vary.