Would this

http://www.trademe.co.nz/motors/car-par ... 877770.htm

be worth getting? and what exactly does it do? what are the benefits?
pretty sure it plugs in to the engine side port on the computer by strut tower.

My gut feeling tells me that it is used to set the CO on your engine.
Do yourself a favour and ask for the partnumber from the fella and check it in the partscatalogue on the BBB website (take the 1985-1991 book and crosscheck it in the numerical register (group 0))