First, Welcome to the board.
Second, yes it is a bad idea.
Violating 18 USC 922(v) is a 5-year felony. A felony conviction prevents you from ever owning firearms again. Having an illegal gun that never leaves the house is not a legal defense -- ask those folks in Waco. Oh, sorry. You can't.
There are several legal options, especially when you state that you may not even be done by next September anyhow.
Time is not the issue in building an AR; you can assemble on from start to finish, including mounting a barrel to the upper receiver, in an afternoon.
Money is the issue with AR's. You have time but not much money. Selecting the parts for an AR can easily take up all your time and money between now and September.
To build an AR before September and stay legal:
Option 1, pre-ban features: Buy everything but the lower receiver. Selecting and assembling the upper receiver and the lower parts and buttstock items will not be pocket change. Then in September, buy the lower and do the final assembly. You can also leave everything in parts form, for there is no "constructive possession" of non-NFA firearms, but the temptation may be too great, and pushing in the pins on an "assault weapon"-styled upper on a post-ban lower is a felony. Therefore, I do not recommend having all of the parts on hand, ready to go together, and the serial numbered lower receiver is the "firearm" portion, without which all you have are parts.
Option 2, post-ban legal: Assemble your first AR with parts that won't become undesirable in September, I.e. an A2 or Ace tubular stock, and a Bushmaster Izzy, or some other desireable muzzle brake. Cjhanging the parts to a colapsable stock and flash hider would be barely $100 on the cheap in October. You can even have the parts on hand -- just don't replace the post-ban parts with the preban ones before the expiration date or again, its a felony.
If you have enough money before September, go with Option 2; if money is too tight, save the lower reciever for last -- installing a stock and the lower parts kit takes less than 2 hours, and that includes reading the instruction manual.
Be patient and stay legal, for if you are impatient and break the law, it can affect the rest of your life in very major ways. Take this as free advice from a lawyer .
Cheers, Otto