Is it possible to change the Head/stereo unit in a Lexus IS 350 with an aftermarket one?

Looks like it wont be to hard to do. the dash is reletively flat and a double din kit is about 25 bucks at a stereo shop.

the whole job should cost about 200-250 to install a radio and make it work. If you buy the radio from the guy putting it in, its usually a better price

The choice of radios is a matter of personal preference. My New favorite is the Pioneer Premier Avic F90 BT. It has Hard Drive Navigation, 3 D Navi maping, MSN Direct gives you nav traffic information, local movie show times and the gas station prices on the navi map (Soo Cooool). built in Bluetooth and iPod w/ video. SD Card slot in front too.

Msrp is kinda high but the radio comes with factory 2 year warranty. Our shop is selling out every week.

Call crutchfield, 1-888-955-6000. They will tell you and show you exactly what can fit in your car with no modification. If you buy from them they will also give you free install gear and a sheet that gives detailed instruction on how to install it. Or you could buy it form a local shop and have them install it. But they will charge a little more for the headunit, and they will charge for install gear and installation time. Also being a Lexus its probably a little harder to do installs cause everything is built so good and tight. I know a local shop wanted $60 to install in my Audi TT. That's why I ordered from crutchfield in the first place. Check them out, they also have seriously the best customer service on this planet.

Yes, its possible to replace the headunit/stereo/speakers in most vehicles today.

It wont be overly expensive, but it will depend on the type of stereo you decide on. If you start at $500 and work up from there, you will do fine. $1000 should get you something brilliant