Labeling

What would be a good printer to use for labelling an existing harness that has the end pins already connected for purposes of identifying which wire is which during a car engine swap.

This may not be preferred, yet I have seen professionals use heli tape over labels for existing harness removal. It is clear, as long as you do not put your fingerprints all over it due to the oils, grips extremely well, and relatively quick to use.

Masking tape and a sharpie

I recently picked up a Dymo XTL 500 label printer. It was expensive, but it can also handle a large range of heatshrink diameters that most portable printers won't (1/4 in. - 2 in.). The cartridges that hold the heatshrink are also fairly expensive, but if used with some thought can last a while. You can also do larger print runs by connecting it to a PC. If you have a lot of wiring to do or have other uses for labeling I would highly suggest it.