Thanks for joining and posting your question...
I assume you have seen the other thread with advise on how to make an AR-15 pistol but if not here is a link:
http://www.quarterbore.net/forums/showthread.php?t=193
Next, you need to understand that AR-15 pistols are a finiky beast and there are lots of guys that have been having problems. One of the best solutions is the Olympic Arms OA-93 upper based pistols. These use a piston system that is much more like an AK-47 then an AR-15 and these seem to work pretty good.
The other way to build a pistol is to use one of the short pistol buffer tubes. The older and original style tubes have been the real issue for many want-to-be AR-15 Pistol owners. There is a newer style AR-15 pistol buffer that is longer then the older system and I understand that it works better but I have not seen or tried one myself... I welcome more advise or a link on these if anybody can help...
My personal AR-15 pistol is a BOLT ACTION as I built it with no gas port or gas tube. This works great for me as here in PA I can not hunt with a semi-auto but I can use a Bolt-Action pistol. Still, it shows how you assemble an AR-15 pistol...
http://quarterbore.com/ar15m16/postbanar15pistol.htm
Now, as for the Mil-Std-1913 rail and slings, etc... There are many options to take care of this so yu really need to decide if you have the money to go with the OA-93 style or if you are going the more traditional route with the short pistol buffer tube....
Another thing you might consider is doing one in 9mm or 22LR. Both of these cartridges are blowback so there are far less issues to getting these to work...