As an electronics guy my entire life (and my father, and his father before him... you get the drift), I'd vote for engine first. Simple reason is that an engine stores better when not being used than most electronic items. Especially if you're building a fully electric panel, you'd want to save buying electrical items for last.
Also, vacuum instruments have much the same issues... unless stored in a clean environment they will tend to gather dust... and as such might be inoperative or have a shortened life once installed.
Now, having said that you should have a pretty good idea of the general layout of the panel pretty early on. Knowing what goes into your panel is actually pretty important as this is the part you'll spend the most time staring at when you're done (especially in IFR). You have to be comfortable with the panel, and planning early is a good thing.
HOWEVER... again a reason to wait. As you finish up your plane, firstly electronics may have come down in price, or there may be new toys you can install... or you might just find that you've neglected an instrument early on that you suddenly have to find room for. This is especially the case after hanging the engine... how much info do you WANT on what the engine's doing?
So my vote? Leave the panel as long as humanly possible. Make it the last pieces you buy, and the last thing you install in the plane. That way you've spent more time with your bird... you know what'll make you comfortable when you fly... and you can plan around what your bird has instead of what you hope it will have.
Wow... my first post to the board... and it's a long one!
