guys some of the info you are posting is not true
1. Presteige (base) with EMV (option)
has ML system and camera etc
2.Sports +EMV
3.SL
and for the bluetooth info it was released in MY13 not 2012 the 2012's you may have seen that have it are late 2012 builts which are effectively 2013 MY13
as for your question
1- What I would really love to do is to install a nexus tablet for a head unit. I haven't seen many guides, and don't know how doable it is (if you're not an electronics know-how)
short answer no but long answer yes if money is not an issue you can do it
but reality there is no easy way
2- Are they easy to install Bi-xenon lights? Maybe track a pair down from wreckers and install them? But how much? Another thing I would love to have are the LED daytime running lights inside the headlights like the newer models. Possible to fit them into stock older headlights?
yes its plug and play a set will cost you around 800-1000
3- Don't know the difference since I haven't compared with or without, But I drive a lot and would love a nice clear audio system. Is it worth it?
all 2006-2009 IS250's with nav have ML system so you probably wont be able to compare if your not intrested in the non nav models
4- Don't know what's involved in installing a reverse camera or parking sensors, can anyone shed some light?
installing sensors are easy
reverse camera is a whole lot different