[..] Basically I'm wanting to detect when people walk up a few starts towards the front door so I can then set some actions in motion.
Would a Rangefinder typical of those used in robotics be suitable or are there other sensors that might work better?
A Sharp PSDs like the R316-GP2Y0A710YK could
be used, but it's $20 and the beaam of detection is somewhat narrow.
An Ultra Sound echo device is another option, but may be hard to find in a ready-to-use version
A LASER fence is fairly cheap to make, but you need to mount it on something firm, to keep it in alignment and it will take some work to setup and fog, dust etc. (any air pollutant of a certain size really) will make it visible.
You could go advanced with capacitive sensing, RADAR or radio field disturbance, but they take some (a lot) experience in electronics, which you probably don't have as you ask.
The PIR (Passive InfraRed) device that the previous poster linked to (or any other PIR unit), will be the best solution for you, I think, as it works right out of the packet, perhaps with a little adjustment of the sensitivity control to suit your project and the field of view can be up to ~180°.
Plenty of other PIR units can be found, both as stand-alone units and built into lamps and alarms, which in some instances may be cheaper, but takes a little hacking into. Some drives a relay, which makes it fairly easy to connect up your own stuff.
How much tolerance on the distance of your "customers" position can you tolerate?