There are a couple other useful threads to check out - trikes vs. bikes
and the three wheelers
threads. Several makes and models get name-dropped, and quite a few awesome custom jobs are shown and talked about. If you take the route of building or modifying your own, check out unjonharley's Bike Builders Unite
thread - part support group, part resource guide, part shared experience and all kinds of awesome.
The kit Elorrum talks about sounds like a great way to go if you have a current bike that you can use as a base.
As for me, my girlfriend and I looked at a few cheap options and came to the realization that if we were going to invest in something more than a cheap junky playa bike, we wanted something better than a cheap junky bike. And a cheap trike is just that. We saved our pennies for a little longer, and got port-o-trikes from Worksman. Worksman is a great bike company out of NYC, actually the oldest one in the US. Their primary business is commercial/industrial stuff for delivery companies and pushcarts and stuff, but they also do some damn fine bikes and trikes. The "port-o-trike" models we went for not only had 3 speeds (so we could pedal through dunes and stuff a little easier), but could fold in on themselves. They'll actually fit within a 3'x3'x3' cube of space if needed. That was a huge plus for us, since it meant we could fit them into our van, and also so that we could potentially keep them in the loft and use them in the city if we wanted to. They cost around $500 apiece, but well worth the investment.