Try to keep the upper and the lower from the same company. If you go PSA, you may need to do a little work to get it reliable, or you might get lucky. There’s been a few that run right out of the box. Buy a legitimate Armalite buffer tube, spring and buffer as a kit if you can and that will function without question. There are other options, but that one has been proven here over and over again. Start a build thread and list up your parts before you order…you might get some feedback that way and it will be a little more specific to your plans. Cheap is a relative term. If you go on the absolute lowest dollar end of things, you might have to do a little extra work, but it’s all doable.