Juvederm or Restylane are probably the best ones out right now. Hopefully something similar will come out soon that lasts a bit longer like 12 to 18 months. Whatever you do, don't use the MICRO-BEAD based injections. Those actually inject little micro beads which can sometimes reportedly clump up over time and mess up your lips! It would be great if something lasted forever but only if it worked properly and right now, that doesn't seem to exist so don't risk it with these types as they can't be fixed too easily or at all.
As far as the amount to use, try asking the doctors on Real Self but they will really need to get a clear idea about what you are trying to accomplish. There are actually different types of Juvederm- Juvederm Ultra and Juvederm Plus, which is thicker and more viscous.