Though, I am not sure exactly what you are looking for, here are some suggestions:
If aesthetics are important to you, find a "generation" that you like. A 99-04 truck has the same cab, 2006 and 2007 are weird looking to me and the 2008 and up cabs all look very similar. I am not sure when the last model change was or if there has been one recently.
Anything after 2006, will likely have some emissions equipment. If that is important to you or required in the country of California, that limits your options on finding an older and cheaper truck to build from. I have seen 2007 Fuso trucks with emissions and 2007 Fuso trucks without. I am not sure about Isuzu.
The Isuzu engines are quite detuned and anything after 1999, will be a 4Hxx series. There is a 4HE and a 4HK and at least one other. As emissions laws changed and new models were introduced, power levels changed from about 175 BHP to 215 BHP, which is the current spec. These motors are detuned for longevity and have quite a bit of power hidden inside, but unlike the USDM diesel pickup market, there are virtually no tuning options for them. It would take some pretty serious cash to run one on MoTec, Zeus or EFI Live.
Transmissions are all from Aisin Seiki. Older models used an A442, AW50 and/or derivations thereof. The newer trucks use an A465, which is the same as an AS68RC or AS69RC found in a Dodge chassis cab (not pickup) 3500 and 4500. Using the 68/69 transmission allows you to mate any transfer case that fits a Dodge HD pickup to the back of it, as it uses a very common bolt pattern that has been around for years.
What do you want to do with the truck? Is is going down a dirt road for a few miles to get to a campsite or mountain bike park or do you want to spend days off roading in Canyonlands away from it all? Do you need to leave NorCal and drive 4-5 hours on the freeway to get into the Sierras or down to Baja? All of those answers will determine what truck and how it is built IMO.
Choosing the gasoline version will give you a cheaper purchase price and less emissions equipment, at least in the SE US, it does., and you will have a few more options related to tuning the engine and transmission. The engine is GM's L96, which is rated at 297 BHP at 4300 RPM and 372 foot pounds at 4000 RPM. It is coupled to a 6L90 transmission. This driveline basically comes out of a Silverado. So the factory transfer case will bolt to the back of the 6L90, after you change to a 4WD tailshaft. If you want something like an Atlas or HERO case, those will bolt up as well.
If you convert to 4WD, you will need to redo brake lines, suspension, steering linkage and more, so it won't be a cheap swap. If you can do this work yourself, I would estimate 10-12 grand. If you are paying a shop or using high dollar parts, I would expect 2-3x that amount.
Custom Springs from Deaver, Alcan, Atlas or Betts: estimated at $2000 - 2500 for all four
NV273 transfer case, rebuilt: $1000
A pair of Superduty axles (2004 or older): $1000 - $1500 for front and rear, may need work
Chevy D60 and 14 bolt from junkyard or CL: $500 - $1000 plus rebuild parts, R/P, etc.
Semi-custom dampers front and rear with mounts and Timbrens: $500 - $1000
Basic fab work, i.e. t-case cross member, spring hangers, shock mounts: ??
Custom brake lines, steering linkage and miscellaneous parts: $500 - $1000
New wheels and tires to fit the 8 lug bolt pattern (Ford or Chevy): $1000 - $2000
Custom driveshafts from High Angle, Denny's, Tom Wods: $500 - $1000 depending on specs.
No labour, no flat bed, no camper, no roof rack, no LED light bars, no comfy suspension seats, etc.
If built right, you will have a very capable truck on and off road that will do anything short of heavy hauling or competition rock crawling. You should be able to travel in comfort at 65 MPH on the freeway and get into the back country with all of your gear or it's not worth building IMO.
Download the body builder's guide for the year of truck you are looking at. It is a large PDF document and will have detailed specs that the websites and brochures simply do not have. If you lift the truck and keep it 2wd, running singles on the rear will likely cause a marked difference in front and rear track width. I do not believe that off roading with a dually rear is a great idea, but it will handle dirt roads and grasslands just fine with a bit of pressure dropped out of the tires.
Hope that helps.