RE Engineers: You dont need to finish the level 1 upgrade before starting the level 2, start it as soon as its available and level 1 will fill up immediately. The higher an upgrade is the more valuable/easier it is to rank up with the Engineer if you have the mats.
The introduction to the next Engineer happens at level 4 iirc.
May need to relog at level 4 to get the invite if dont want to go level 5 for any reason or short of mats.
Light life support is the cherry on the icing on the cake, can do an explo trip to Colonia and unlock them then.
Optional extras you may want to fully explore and interact with Horizons - SRV bay or double, recon limpet, every scanner you can fit, a small Pulse or Beam Laser, some people have a mining laser for mats (no refinery needed if just getting mats) and I even have a fighter bay just for fun. All of it affects the jump range but its still my 'exploration' ship. So depends what you want to do when you get there
Edit: no fighter on Asp obviously