I guess its a matter of how determined you are to save yourself money. If you have a free day or two to spend working on it then it makes sense to do it yourself and chalk it up as a learning experience. If you're more about convience, then I would pay a professional who warrenties their work. Personally, I'd stay away from purchasing alarms/remote starts from ebay because it often makes warrenting the product difficult or impossible. I often run into a situation like yours when somebody will bring in an alarm and want me to install it, and unfortunately I'll turn that install down. I've learned the hard way over the years. If you're set on using the unit you have, then most likely I see yourself either installing it yourself or finding a qualified friend or aquantence to put it in for you.
If you do deside you're going to do it yourself, you'll want to remove the aftermarket keyless entry unit currently in your car. Be prepaired to soilder to extremely fine wire in the VATS harness when you build the bypass relay. Use a mechanicial pin switch or a mercury switch inside the engine compartment as a safety feature. Identify and test all needed wires inside the vehicle before you even take the unit out of the box. Prep the alarm, meaning cut away unneeded wires and group wires together running in similar locations... the idea is to do as much work as possible outside on the work bench versus laying in the car. Know how to use a multi-meter well, you'll need to know how to check for DC and AC voltages as well as resistance (yay for the VATS system). Stay away from scotch locks/vampire taps. Those are for hacks, causing an ugly looking installation not to mention making the wiring connections even more obvious for a would be thief if they were to break into your vehicle and attempt to disable the alarm. For most wiring connections I'd suggest stripping 1/4" to 1" depending on wire gauge and poke a hole through the center of the exposed wire. Strip several inches on heavy wire, less on thinner wire of whatever wire you'll be attaching to. Run the wire through the 'eye' you've created and wrap the end of the wire around the exposed portion of the original wire in the car. Soldering the connection is always a good idea, and use a good quality electrical tape... I'd recommend 3M Super 33 or 55. And remember, the whole premise of electrical tape isn't for it to be sticky, its supposed to be more so stretchy. Cheap tape doesn't stretch nearly as well. Some guys like to finish off the connection with a zip tie around the collection of the two wires and tape, to help reduce stress on the actual connection if pulled on and to keep the tape secure. Hope you can put any of this to good use.