Hi Steve,
Do you like the Digitrax throttles?
For many folks the handheld throttles can be a deciding factor simply because you may strong feelings about the design from one brand or another.
For example I don't happen to like the Digitrax throttles even though they are quite capable.
I dumped my Digitrax hardware in favor of a Roco Z21 for a number of reasons.
One is that their handheld suits me better but as I said that can be a very personal thing. The are also bringing out a WiFi version of their handheld.
Two if I am in the mood I can use a tablet or a smart phone for a "touch screen handheld throttle using the Roco app for this purpose.
Three the Z21 has a LocoNet network interface which is a very good thing for a controlling turnouts, signals and occupancy sensing.
In fact you can use Digitrax throttles with it.
There are many products out there that are LocoNet compatible and you can use them with the Z21.
Suzie X mentioned the different types of occupancy sensors.
Current sensing with transformers is the way to go for a couple of reasons and if you go with the RR-CirKits WatchMan the price is right in line with the popular Digitrax BDL168 which uses diodes.
One price I saw on the BDL168 was $120 which works out to $7.50 per sensor.
The current price of the RR-CirKits WatchMan with coils is $60 which works out to $7.50 per sensor.
The reason I like transformer sensing is there is no voltage loss AND the block power wiring is totally separate from the occupancy sensor wiring (other then one block feeder wire having to go through the transformer coil.
As to a computer interface device - if you are going with Digitrax the newer units like the DCS240 have a USB connection built-in. For the ones that don't have that you can use a RR-CirKits LocoBuffer-USB device.
If you go with the Roco it has a network connection built in and nothing else is needed.
If you are feeling very adventurous you might take a look at a Digikeijs DR5000 - an amazing unit with LocoNet, ExpressNet connections and more.
The one "downside" is that you have to use some other brand of handhelds as Digikeijs is not making any handhelds to date - though they are working on an app for tablets/phones.
http://www.digikeijs.com/dr5000-adj-dcc-multi-bus-central.html
Feel free to ask any questions you may have.
Frederick