Seems legit.
* 2003 is the best year for the 7.3psd.
* 150k miles is pretty low for that engine.
* The 4x4 kit and lift are worth about 12K brand new
* the pop top will probably run you about 8k
So all in all you'll be in for about 25 to 26K.
Also keep in mind that while these engines are super reliable, they ain't cheap to maintain. An oil change will run you between $80 - $100, and the most likely thing you'll have to replace in the relative short term will be the injectors which I've had quoted out at just over 4k to replace all (including labor).
One important thing to note - if the body is damaged in any way, and you will want it straitened out.. body work on these things ain't cheap... they're essentially one big panel, so any chain body shop that does mainly insurance claims are going to write it off as a total loss for even minor dings on the main body. I know... I just had someone tell me they flat out wont fix a ding in my rear fender. I now have to learn how to do my own body work
That being said, I flew from Denver to OKC to pick up a 1999.5 PSD with zero bells and whistles for $5k. I'm sinking nearly 20k into upgrades... it's more of a labor of love, so if you're truly passionate about the idea, and the van is in good shape, I'd say snag it (but try and talk them down a thousand or so

)