Workbench Refurb

This is a bit of an open ended vague question.

People here have completely rebuilt printers. so if you can search for and find a project log that would help you. 

What  problems are you having with the extruder / hot end.
What are the issues with the Z axis.

Do you have any prints to show  good bad or ugly can you post photos of the print(s)

Th machine may just need a good going over . tighten the grub/set screws adjust tighten the belts.  level the bed adjust the z stop,
calibrate e calibrate xyz   and get a good set up in your various config files. IE adjust software settings.  You may also need to ajust your v ref pots on the board.

tin

The Z axis on a Workbench is finicky to say the least.  The root cause lies within the cantilever design.  Couple that with bushings on rods that may not be true and you have a recipe for failure.  What you're experiencing is what I call bed hop.  Basically the motor and rod are turning properly.  Because the tolerances of the nut on the threaded rod are loose, it can turn a couple layers without the bed actually moving.  Once the rod turns enough to force the bed down the bed platform pops loose and falls until the weight is once again resting on the nut.  With this system having no backlash nut, it relies on gravity to do the work.  The motor and threaded rod should not be pulling the bed down (on this specific machine).  Gravity should smoothly pull the platform down as the threaded rod allows.  The issue is resistance of the bushings on the guide rods.  Particularly the space between the uppers and lower bushings create a situation where instead of sliding they are really trying to bend the guide rods.  The more weight and resistance the bigger the problem becomes.  The good news is, there's a few things you can do to help.  First would be the obvious, clean and oil the rods.
Second, actually rotate the rods in place and watch to see if one is bent.  Sometimes simply rotating a rod will free things up.
Thirdly would be to come up with a way to actually pull straight down on the platform at the very rear or even behind the rods.  (Think small doubled back bungee cord material). Attach one on each side of the platform right next to the guide rods at the top of the platform.  Then go through some sort of loop hook on the bottom of the machine that the cord can slide through.  Then run back up and attach to the top of the frame of the machine.  This will help keep the force consistent through the entire range of the z  axis.  Alternately you could hang some form of weight at the same location as I described above.  One on each side, farther behind the guide rods the better.

Either way, been there and done that.

Hope that helps and good luck.