Congrats on the forum join.
#1 Get a lower...Funshows are great for looking and the occasional deal. Just watch the pricing. Just remember that you can buy a lower for $50 at PSA and $60 for a Surplus Ammo at AIM surplus, but you will have to pay an FFL to receive it for you. Call around - Some are $10-15, some are $20-25, and some more. Call around or Google FFLs in your local area...There are sites where they post their rates - It helps.
Parts kits. There are companies that make things easy and possibly cheaper for you. LPK - Lower Parts kit. Cheak out PSA...They have some great deals with stocks. If you want to buy each part...Probably more expensive in the long haul, but definately do-able. UPK - Upper Parts Kit. Starting to see them at the same places. Not many parts for an upper, but it's nice to not forget a roll pin and have to call Citizens Armory to avoid the $hipping costs.
Cost. You can piece the thing together as you go, but watch your shipping costs. With the price of diesel, shippers are passing this cost on to you. Most companies have a $150, $200, $250 free shipping. Try and lump all your parts together and make larger purchases. If you want to keep the shoe string budget, just watch the parts. Funshow parts are generally not so much name brand - Good? Maybe! Fair? More than likely. Bad? Possibly.
Homework. My words of advice...Figure out EXACTLY what you want to build. Find all the parts you need online. Create a spread sheet for ordering each one individual also include shipping. Then pick 1-2 with the $ for free shipping and create another spreadsheet and compare...You will find the 1-2 is probably cheaper even though you are paying $5-$20 more for this or that. Then, you can look around at the Funshows, Local Gun$hops, and other...At least you know the price of what you want and the shipping on it since you did your homework. This can help you put thing together reasonably. Big picture - STICK to the purchase PLAN! Otherwise, your $700 project becomes $1000-1200 quickly.
Complete kit. Some purists will pound you for buying a complete upper, lower, or rifle kit. Big picture: Who cares...As long as you are shooting. Just don't go posting that you built the thing when you didn't - It offends some peoples delicate nature. Learn the rifle, shoot the piss out of it and modify it if need be. However, you can get the parts you want if you build it yourself. If you are going to buy a Del-Ton or PSA all in parts...Look at the complete upper or Rifle kit - It may be as cheap or cheaper. Again, tinker and learn. You can always build the next one.
Optics. Whole nother ballgame. Build the rifle and put on the BUIS or MBUS type. Then, start the search for what you want in optics, and this game can get expensive quickly.
Flaw. The only flaw I see is you need money to start the project. You MUST have the lower receiver shipped to an FFL unless you buy at gunshow or a local $hop - Takes time. The rest CAN be sent to your front door - I know, it seems odd. Longest lead time is generally the barrel. You need a few general tools for the lower, but if you assemble the upper...It gets to be more in cost for tools. You can learn tricks on this forum to avoid a bunch of tool expense.
I am posting two items...Videos and a Tool/Parts listing. NOTE: You do not need all the tools in the videos.
Videos
PDF
The latter helps you not post your build specs asking everyone to Q/A everything - Just follow the checklist.
Good luck and post pictures when you are building/complete.
Rob