Reliability Upgrades for the AR-15
The AR-15 is America's most popular rifle; and with that popularity there has been a lot of different manufacturers making various different quality AR-15 rifles. Any AR-15 can be made into a reliable AR-15 without spending a ton of money.
There are a few different things can effect the reliability of any AR-15 regardless of the price and quality:
Cheap poor quality ammunition can make any firearm have problems. Ammunition is just like anything else, you get what you pay for. You don't need to spend a lot of money to get good ammunition that will go bang every time you pull the trigger. Some of the cheap Russian steel cased ammo such as Tula is under powered, and will cause many AR-15s to have cycling problems. Spending just a little more money can get you Wolf Gold or PMC which should work fine in any AR-15. Use the cheap under powered ammo for working on malfunction drills.
You don't need to spend much money to get good quality magazines. Poorly designed magazine's can cause malfunctions in any AR-15. Magpul's PMAGs are incredibly reliable. There are many other good quality magazines avilable from other manufacturers. It's not a bad idea to buy only one or two of a specific kind of magazine to make sure they work correctly in your rifle before buying more.
Bolt Carrier Group
If a part is going to break on your AR-15 it most likely will be something in the Bolt Carrier Group. The bolt is under a lot of stress. In some of the lower end AR-15s bolts and bolt carriers have been known to crack or have catastrophic failures after only a few hundred rounds. Most of the cheaper AR-15's do not have bolt carrier groups that were High Pressure (HP) or Magnetic Particle Tested (MPI). If your bolt carrier group was not at least MPI tested it's a good idea to get a new bolt carrier group that has been tested so you know its free from cracks. A good quality bolt carrier group costs $100-$150 and while you're are it you might as well get a Nickle Boron coated or Nitride/Melonite treated bolt carrier group which both have lower friction coefficients and are both much easier to clean than phosphate bolt carrier groups. Don't forget to use some sort of gun oil on it.
A heavier buffer than the normal H carbine buffer can help by adding a bit more weight behind the bolt to ensure it goes into battery. This will especially help after you have fired a few thousand rounds through the gun without cleaning. However a buffer that is too heavy can cause cycling problems. It may take some experimenting to see what the heaviest buffer weight is that you can shoot reliably in your specific AR. A heavier buffer also has the added benefit of slowing the cycle rate which will help dampen the recoil that is felt by the shooter. If your AR has a carbine length gas system you can probably use a heavier buffer than an AR with a rifle length gas system.
Field Repair Kits
If the quality of the lower parts kit is in question a "Field Repair Kit" is a cheap bit of insurance. Typically a Field Repair Kit contains parts that are somewhat prone to breaking or typically lost. Most of these kits are around $30 and some are even less depending on how many parts are in the kit.