For the PSA cars, the only bomb-proof solution for successfully changing the software parameters on different car's computers is by using the dedicated PSA Evolution tool ... can't say about exact prices, but they're usually in the 100 GBP range ... the tool looks like this (some other connectors might be provided, but the picture shows the minimum what you need) ... and you use either LExia or DiagBox (newer generation software) with the PSA tool:
The usual radio/mp3 unit in these cars is either RD4 or RD45 (small hardware differences, the latter should have BT connectivity). Like I've said, you can put whatever else compatible unit inside, if you have all necessary wiring (this was explained in detail in my navigation topic for the two provided nav units) ... in case of the RTx devices, it would be possible to change their VIN code ID to match your car without the diag software (a small piece of software called MiraScripts does this without issues - in case you don't match the VIN code, the unit will work in the car, but you'll get a warning beep every 10 secs or so) ... but if you'd like to enable the instrument panel navigation page, you can't do that without PSA XS ... so, either you get the tool yourself, or go to the dealer or other garage that can do the programming for you.
The nav units that can be installed are (in no particular order, rough descriptions):
RT3 - not actually sure about this one, but it might be possible
RT4 - compared to the RT3, it has a higher resolution screen, on board HDD, 3D maps and radar warnings
RT5 - similar to RT4, but has BT phone connectivity
RT6 - BT phone + music streaming, no HDD for music, maps are copied via USB to the internal storage, like on the previous units
NG4 - 3D Navidrive, DVD movies player (can't be used during driving)
RNEG - similar to the RT6 (the interface is different), no BT music streaming, maps are put on the SD card and inserted in the device
SMEG/SMEG+? (the latest touch screen variant, the type which is provided with C4 Picassos etc)
The GPS antenna can be installed behind the dash, you'll get good reception. The USB cable and socket to the central handrest are an easy job. The rest of the cables, depends on which exact unit you'll go for. Consult my navigation topic once again, it is explained there.
I've never managed to find any photo sensitivity option in the diagnostics (I believe I've seen almost all of the available options in it), so it is up to the basic electronics of the sensor itself ... the last couple of days the DRLs on my car are on even when it is foggy and you'd expect to get dipped beams, while on some other cars (different manufacturers) they're not, so go figure ... one "funny" thing I've figured out is, if you're driving on auto lights, with the dipped beam on and front fog lights, if the auto function will switch to the DRls, the fog lamps will remain on ... wouldn't figure it out if I hadn't noticed something "not-logical" on the instrument panel icons (sidelamps + fog, without the dipped beam) ...