Most "AR Kits" come with the barrel installed on the upper receiver, head spaced and ready to go.
You will need some simple hand tools, some punches will make the job easier. The only "special tool" you will need is the stock spanner wrench, if you get a CAR/M4 collapsable stock vs. the A2 solid/fixed stock.
You can pick up a kit for @450-550 and a lower receiver for @110 and be done for 660 or less. More money will get you some "up grades" like flat top gas blocks, or different style barrels or such. Or you can spend more $$ for "better quality" parts.
Getting a complete upper (you have to ensure that it comes with a bolt, bolt carrier, and charging handle, to be a "complete upper") will have most of the technical building done, and you can mount this on most any complete lower.
You can get a lower kit (to put on your lower) for @$50-60 and a stock for @$50-75, so you can add up the cost.
This process is quite easy and within your capabilities. You can get the AGI AR15 tape and it will give you the information you need to assemble a kit, or you can search out someone locally (there is a thread with folks willing to help here somewhere) that are willing to help you a little.
Good luck, you will have fun on the process.
By the way, if you are going to build just a basic AR15, you will only save $75-125 over a complete built basic Bushmaster, so it may not be worth the hassle. However, if you are going to specialize a rifle, you can save more.