I can't remember everything we looked into when my dad got his but there are a few things you want to think about.
First is gas or electric?, personally I would go with gas powered, they're more powerful and more portable. Its bad enough dragging a hose and all out, you don't want to be snaking a stupid electric cord through the puddles your walking in on top of it all.
For gas, you can get some mighty high pressure. 2000psi is probably good enough, I think my dad's is 2400. What you really want to look at is the cleaning factor (I think thats what they call it) something like the volume of output X the pressure. The higher the better.
You don't have to worry too much about damaging things with it as long as your careful. If you get a high pressure machine like 2400 psi and set the nozzle on pinpoint it will blow the pulp wood out of your deck, take the pin stripes off your camper, and leave pits in anything plastic you point it at. The simple solution is to not run it on pinpoint on these types of things. Having the higher output and pressure is really nice when you run it on a wider spray and it will really clean stuff up nicely. Then in the rare event you want to strip off paint or really clear off a peice of concrete you'll be glad you've got the high pressure and can put it on pinpoint.
I can recommend one by Devilbiss, thats what my dad's is. Has a 5hp briggs and stratton engine, a pretty decent wand, and is nice and portable. Had it several years with no problem. You can set the wand on low pressure and it will be sort of like a sprayer head on a garden hose, but has a siphon tube to spray soap or solvents through it to pre soak stuff. Then you kick it up to high pressure and it blows straight water with an adjustable spray (spread, point point etc.) to blow things off.
The one by craftsmen would probably do just fine. They're not a real complex machine, so I doubt you could go wrong with a gas powered one from a reputable company. The electrics I'm not sure about, they seem kind of weak.
