Sorry for the extremely late reply. Hopefully by now you have grown impatient and decided to tackle your project. In case you haven't here's the scope:
Stand alone remote starters are avaliable from the manufacturer (for most cars) and aftermarket. Directed Electronics (DEI) has many of these systems that simply give you the remote start option. Of course there are many otions avaliable like two way remotes, automatic timers, various ranges, etc (I would go with a DEI remote start as I have found they are pretty simple to install as compared to other alarms such as Audiovox).
The install would be a bit easier since you would have to stay on one side of the vehicle (drivers side under the dash at least for e46) except if you wanted to connect the hood pin shut-off switch. Then you would have to go into the engine compartment or the car's computer. I don't know what type of unit you are looking for, but I have read in various treads that there are certain harness avaliable for certain alarms, but I don't know if this applies to remote starters as well. If you were looking at a unit that does not plug into a factory harness, then you would have tap into (not splice or cut) 5-8 wires for the remote starter then cut or spice (not tap into) into 4 more wires to get the remote starter running. Since your car is 95 or newer, you would have to use a bypass module (DEI 555B, DEI 555U, or equivalent) and have an extra key to place inside the module so the immobilizer system allows the car to start. This would require the use of accessory channels in your module such as the "ground during crank" wire. It sound pretty complicated but it actually isn't if you have any questions, and I mean any questions (you or any person reading this thread) email me at firstname.lastname@example.org
. I check this thing religiously. I am alway happy to help someone out.