Two choices - one is save up money, and parts you're going to use regardess, and use existing stuff. Items like supressors, optics would do.
Magazines, Ammunition are ALWAYS a good idea if they fit your budget, so if you're stumped, start there. FDE MagLevel PMags with Ranger Plates are simply the best magazines being made right now - I have black ones, and they're just plain awesome.
Basically, get a lot of the stuff you can use until then, you'll be happy sooner, and also have an already proven kit with equipment that works and has been tested.
Other option would be to get an AR-15 in the color ACR you want, and then kit it to the max, knowing that you'll replace the base unit anyway.
MIAD with GripCore,
Optic w/ Mount
FUG (or TD/KAC VFG if applicable)
TROY Rear BUIS (until the MBUS rear's are out)
I'd honestly wait on the barrel - those will likely be the hardest to source (piston conversion components for existing barels)
If you're can shopping - pick a supressor with dedicated FH (Gemtech, AAC, KAC) if you want, but tax stamping something you don't own yet is a bit of sunk cost early.