If money is an issue, then I guess getting the finishing kit first would help.
But a brand new engine stored properly and kept lubed and dry will probably not have any problem at all even for years.
I've heard too many reports of people taking properly stored engines out of mothballs, pulling the jugs, and finding that it looks like new with no corrosion whatsoever.
With that said, the bigger question is whether to get the finish kit first or all the radios because for most models, you have to install the aluminum panel on the fuse in order to install the windscreen.
So to give a totally opinionated opinion which needs to be followed by nobody, I would get all the radio trays and wiring harnesses, compass, chronometer, etc., and build the instrument panel to the point to where all you have to do to finish up is to slide in the radios themselves.
Then you're ready for the engine so you can hook up all the wiring for it and the controls still with a lot of room to work behind the instrument panel.
At that point, you're ready for the canopy installation and cowl at the same time so there might be an argument for getting the finish kit, engine, and prop at the same time.
With my radio installation trays and harnesses on the way, the finish kit here, and the engine to be shipped in about a month or so, that's my plan anyway.