Hi, and welcome to the club.
I can answer a few questions for you, although I am not a club moderator.
1: This would be fairly easy to fit, if it's like any of the interface boxes I have seen beore. Normally you would have to cut 4 wires which run from yout Nav CPU to the front monitor to obtain the additional picture input to your front monitor. My main concern is how this box would transfer any form of audio into your oem system.
2: The module would actully work over here, but if it states there is a built in TV tuner, it would be advisable to check whether this is PAL-I system, as this is what we use here to obtain both picture and audio in the U.K.
3: There will be some drawbacks, as like most car dealers manufacturers, they only like you to pay through the nose and buy there equipment and have it installed at the manufacturing dealer, there will be an electrical warranty issue when you tap, or break into oem wiring into any car.
4: I have seen various products on the market, BUT the USA products seem to be like sitcher units, some with built in TV tuners etc, I have even seen a PIONEER TV input/TV tuner converted via an aditional adaptor which is all used to obtain both video inputs to your monitor as well as additional AV input switching like DVD,games console etc... ( read my note on the bottom on the BMW oem system )
5: It should not effect your reversing camera, as this utilises the additional video input already incorporated into the OEM lexus system which works on a "exciter" wire, i.e what ever you may be viewing on your monitor, as soon as you put the car in reverse, there is a feed from the reverse light relay system triggering the AV box to change the video input to reverse camera.
# consider also the following, the BMW eom system only allows you to view a TV picture whilst stationary, one reason for safty, the other is- whilst the car is moving the TV tuner repection is beyond any reasonable quality, back to the good old days rule when you placed a 14" TV in your kitchen and could only get 1 or 2 clear channels and have to twist and turn the aerial to obtain a clean picture on the other channels. The car is the same ALL the time, as this "aerial" is always moving, thus a poor quality picture, there will be shortly DIGITAL TV incar tuners available which would offer a better performance then the current analogue systems.
BMW Oem AV system notes.
The BMW with oem sat nav / tv tuner actully offers the following.
1- 1 video input which can be used with an aftermarket reversing camera which also has an input for the "exciter wire" ie, you can connect a reverse camera to this input and switch from a posivtive feed from reverse bulb to negative feed into a PIN on the AV box to offer simular features like your LEXUS oem system.
2- The BMW AV system as in point 1 above, also offers an AV input at the AV box, however, to utilise the picture to be displayed onto the front monitor an additional bypass harness needs to be purchase to overide the can-bus speed pulse sensor to allow you to view whilst on the move.
3- The BMW system as in previous 2 points, also offers a "true" AV input, where it has the ability to accept MONO audio inputs, so when the AV source is selected via the TV tuner it not only displays a picture but also MONO audio through the oem speaker system. The Lexus does not offer this as far as I know.
I hope this all helps...