Getting the extruder correctly designed is by far the hardest part of building a printer. Most take there chances with purchased extruders with mixed results.
I'm using an all metal design that uses PTFE. The trick to using a PTFE liner is to fully support it in all directions. At first I tried brass but the filiment kept sticking. There are a lot of theories/claims as to the best way to build an extruder. Do a lot of research and don't just go with what seems to be popular.