3" is a lot of lift on a VX. Many have done it, but consider its use as a daily driver. Most drivers so inclined do just fine with 0-2" of lift, easily obtained with springs and a t-bar crank.

There are a lot of 18" tires out there, just few that neatly fit the 7" rim width that the VX has.
Most owners will steer you in the direction of the OME 912 lift (search 912 +Triathelete) and the Nitto TerraGrappler tires, size 285/65R18.
This combo will increase the height of the VX by almost 3" overall, plus the wheelwells will be filled and the truck will get a wider stance. It's both a visually appealing and functional combination.

As for the "onboard computer," there is no crack for the Isuzu ECU. You'll have to manually compensate for the speedometer error unless you get one of Tone (Monday's) speedo correction gizmos.
The ABS will survive a 10% change in tire diameter without too much trouble.