I agree with Airfox, that the fuse is a better place to start for the reasons he gave. However, I did start with the wings first (not quick-build) because they are much less expensive than the fuse kit. There is a lot of work in building the wings from scratch so I did keep myself busy for quite a while before I need to rig them on the fuse. You can't finish them up and cover them until you have the fuse. The fuse assembly seems to go quite fast so you pay more money and don't keep yourself busy as long as the wings. All in all I had no problems in starting with the wings first, just be aware that they will be hanging around in their skeleton form until after you get the fuse. You can rig the wings and then finish them up the first thing after the fuse comes in the door.
The painting step comes next (after covering everything) and it is quite a chunk of money, more than you would think, around $3000 if you go Aerothane, for paint and supplies. Spray equipment and paint booth construction are extra.
The FWF kit does you no good without the engine so these two items add up to quite a chunk of money. I guess you could fit the firewall itself, the boot cowl and the engine mount without the engine, but these items go quite fast.
You could also get started on the instrument panel layout and some of the fuselage wiring before aquiring the engine, but you really do need the engine and all its sensors before you go too far in wiring the panel.
So yes, you can pay as you go which is what I did. But if you are like me, the farther I went the more and faster I wanted to go so I could get flying the little jewel, so all these chunks of money kept flying out the door at a pretty good clip.
In my opinion you would never want to buy everything at once, especially the engine and avionics. In the year or so it takes to build up to this point many new things can happen with engines and avionics and you don't want to be stuck with old stuff that has just been sitting around while the world moved on.
Jim