Avoid the cheap coilovers (basically anything under about £600).
If you only want to lower it then springs is the best option. Anything from ebay random make springs around £70 to £170 for eibach pro kit.
It all depends on if you care that it drives well or you just want the look. If you want it to handle and have the lowered look the most recommended kit is blistein shocks (B4, B6, B8) and the eibach springs. You will need to budget at least £400 for this and thats before fitting and if any bushes, bump stops, dust covers etc need replaced.
On a cab I wouldn't go lower than -40mm as it will scrape the sump and exhaust on the smallest speed bump.
Here is some pics of my cab on -40mm GMAX springs and standard dampers:
http://www.e30zone.net/modules.php?name ... c&t=216828