> but its slow speed and fuel consumption is a downside for me
The KAT1 can do 16r20 tyres. That will add to the top speed and the off road capability. Also - The axles are somewhat standard, and can be "upgraded" to fast axles.
> The L2000 also looks good but I need an extended cab for my 2 kids
The L2000 cab is very similar to the Steyr and the LMTV. It is for three people. But the middle seat is not the most comfortable for a full size person. For a kid it might be OK. The KAT1 can easily seat 4 people in the cab with comfort.
> Could you show some pics of your expedition vehicle I am always looking for ideas.
Sure - not the best pictures. My expedition vehicle is nothing like a Unicat. I simply don't have the money. It's more like a traditional camper build on top of a small 4x4 truck. The gallery is on Facebook:
https://www.facebook.com/linuxjoergen/media_set?set=a.10379908323.18107.724423323&type=3
> The Mercedes 1017 is a nice platform but not as off road capable as the steyr or lmtv.
Not quite - But one can replace the duallies with bigger single tyres. It makes quite a difference.
http://www.expeditionportal.com/editorial_2011/GXV_Mercedes/MB-10171b.jpg
Regarding offroad capabilities in an expedition vehicle - Seldom you need real tough off road capablities. More like real tough road capabilites. And fording depth as well. Maneuverability is just a big a issue. Long wheelbase or what else might cause a big turning radius is a no go... This for instance is a joke:
http://farm4.static.flickr.com/3149/2867303224_d3c2f71ebf.jpg
You might have big roads locally - but if you want to go everywhere, you have to consider the lowest common denominator. The above truck would have a hard time getting around in for instance Norway.
> pass through or just be able to put on a longer shelter.
Note that pass through requires to be flexible because of chassis twist. Like the pass through on articulated busses.
Getting parts is of course an issue. If you travel mostly in US, a US truck would be the preferred choice. And also: US trucks are build for US conditions. More comfort with conventional style trucks. In Europe and some other places you would need a COE to make the wheelbase short. In Europe, Africa and russia, European trucks are most common. In south Asia they have local brands as well.