It would be wise to decide what configuration you want before you buy. Some stuff is simple to change, others are better purchased at the start. PSA lowers are good, have mil-spec buffer tubes for stock compatibility, and can be had with the Geissle SSA-E trigger. I just bought that model on sale for $300, a good savings.
As mentioned above, a mid-length or even rifle-length gas system is preferred for smoother shooting, and possibly less wear on parts.
Look for good parts in an upper, bolt, carrier, etc. This video series has a lot of good info on what matters and what doesn’t:
