The guys I have dealt with will move into a truckmount when they get busy enough that being more efficient will net them more profit.
You can and will be able to clean more carpet per day/week/month with less labor than with a full time use of a portable.
Align yourself with a local distributor whom you can trust. One who is interested in building a long lasting relationship.
Don't buy more machine than what you need. If your just doing residential carpet don't go out and buy the 23,000 dollar machine when the 13,000 dollar might be all the machine you'll need.
Spend as much as your budget will allow on time saving and labor easing accessories. Such as an electric hose reel or a water tank. Make sure everything has a specific place on your van. Buy the block and tab holders, in line sprayer holder, wand clips, etc.. Organization will go a long way.
Sure you can buy a great machine built in someones garage that takes up 3/4 of your van, but why not spend a little extra money and buy a simplified space saving machine. Make sure there is support after the sale. Not just from your distributor but from the Manufacturer as well.