my keezer is very similar to GrainToGlass'. starting with only the freezer and a couple picnic taps is a great idea. you'll want some decent moisture control inside the keezer since you'll need to open the lid each time you want a beer. go for a two body primary regulator right away, this allows you to carb two beers at two different pressures and sets you up for future expansion. once ready to expand, put the collar on but only the rough lumber with insulation, add the fancy finish boards later. paint the exterior of the rough lumber if you don't want to stare at naked wood for a while. you can add faucets as you go, no need to get them all at once. you can either pre-drill holes for all the shanks and plug the holes with foam or similar, or drill them as you go (messier). the bigger up-front purchases will be the secondary regulator assembly and the drip tray, you'll want to buy those only once so get the size you'll want for the final install. maybe go with the drip tray first and a couple faucets, basically expanding the picnic tap system. you don't need the large secondary regulator assembly for that.
and get the biggest co2 tank you are planning for right away if you can, no need to buy a smaller 5 lb tank and then have to upgrade later to a 10+ lb tank. might not be a bad idea though, to have that smaller tank as a spare. i started out with two 5 lb tanks but then quickly jumped up to a 20 lb tank. i use one of the old 5 lb tanks during kegging for transferring sanitizer, purging, etc. the second one is a full-blown spare in the event the main 20 lb or smaller 5 lb tank goes out.
