My Hilux was pretty easy, although it was a YN57, so a little newer than yours (although I think many parts are interchangable??).
I Just got the under-dash unit and the on/off dash switch for under $200 from a big 4WD wrecker (although my hilux is 2WD), which were a 1/2 hour job to fit. To fit the under-dash unit, you just remove a fill-in piece of ducting from under the glove-box. The amplifier comes with it.
The wiring may be a bit awkward, but it is really just a matter of getting your dash switch to tell the ECU that you want A/C.
The condensor can be an after-market job, as can the receiver/drier and hi/low cutout. Getting it plumbed and gassed should be your biggest expense (some of the fittings are exxy), but you should get out of the whole deal for under $1000, and it will be better integrated than an aftermarket kit.