What are the things to look out for when buying a motion detector?
Well is it to be used indoors or outdoors? Because a lot of the technology is generally used for detection indoors. That is not to say you do not get them for outdoors it's just in my experience the ranges are more limited.

You get detectors that work on four different technologies they are:- Passive Infared (PIR), Microwave, Ultrasonic and Tomographic. The last two are more uncommon in the general market. More and more these days you will find the first two technologies have actually been combined into a single detector commonly known as a Dual-tec the idea is to limit false activations.

Apart from the detection technology detectors are based around two types of usages. One type of detector tends to be used as an activator for mains voltage lighting. And are commonly fitted these days for reasons for saving power as they switch the lights off after a person leaves the area. The second usage of these detectors is for intruder detection. Since you seem to be interested in activating a light and or sounder I would suggest using the low voltage intruder detection version as inside of the device they commonly use an alarm circuit and a tamper circuit and a 12v power source. The alarm circuit is normally a open/closed circuit which can be made to do what you so desire. You can run the sounder and light directly off the device as long as it's low voltage and low current usage or within the detectors specification however if not you can stick in a relay that will do the job. This option will most likely be the cheaper option also

Another advantage of low voltage intruder detection detectors is that quite a few have ability to adjust the detector range this is normally done via a dial which takes the form of a variable resistor built onto the printed circuit board of the device. Some manufactures also give you interchangeable lenses for the front of the device which alters the spread patterns of the microwave/infared radiation. This can increases or decrease the height and width of the beams. Another thing to watch out for which is not hugely important but can and i have seen it cause issues is that if you have multiple devices and they overlap make sure that you are buying different frequency devices. By that I mean that a manufacture will sell multiple versions of the exact same device the difference is what frequency the radiation is put out at this is designed to stop devices interfering with each other.

I have tried to cover all the bases with the little information you provided. If your after more detail on anything just let me know, I have extensive knowledge in this area.

Edit: After reading your post again I noticed the key word "Portable" this could change the outcome entirely. Could you explain the usage of your device so that i can revise changes