I've been installing alarms and remote starts for years. I few points I'd like to mention:

Though I haven't worked on my Crossfire yet, the documentation I've read proves it to be more difficult than most cars. Mostly because of the location and tight spaces of things. So expect to pay a little more.

I live in NY/NJ area and the most you'll want to spend labor wise for an alarm WITH remote start is $250. And that's if your going to a reputable shop that backs their work. The units themselves go from $100 - $500.

The Viper 5900 you referred to is a great piece that should cost you $450 - $600 installed depending on where you go. Although it may seem cheaper for you to buy that Viper on a website, you are better off getting it from the place you install, mainly because of warranty issues.

When that rep said Code Alarm was better than a Viper I almost crapped in my pants. The only reason he mentioned that piece to you is because he'd make double his money. Where that Viper my cost them up to $250, a comparable Code Alarm (made by Audiovox) would cost them under $150.

Though other brands may have "cool" features that Vipers lack, its not worth putting any piece other than DEI in your car. By the way, Installing a $20 remote starter is no different from installing a $600 one. The labor is roughly the same.

Don't worry about voiding your warranty by going anywhere other than your dealer. Obviously if they screw something up then you are void, but going somewhere else will not automatically void your warranty. When a dealer does remote starts they are paying a local shop to do the work anyway, so you end up paying double. I don't know any dealer that will offer a lifetime warranty on remote starts so you want to go somewhere that does.

Last point, VERY IMPORTANT. Find someone who knows what they are doing. Don't talk to managers, talk to the installers. Ask specific questions about your car to see if they know what they're doing, like "diode isolating" and "running tach-less".

Hope this helps. I do this for a living, I'd do it free if I didn't have bills to pay.