Wow .. long message to start off with in this forum ..
First thing .. welcome
Second .. take your time with the Jeep .. don't go in to it too quickly.
The easiest thing to do is to research the different options available to you, and to talk to some of the people who have already completed the projects that you wish to take on.
Now .. as far as seats go, I used seats out of a LeBarron GTS .. made some brackets .. they sit nice .. but very tall.
As far as tires go, the best thing to do is run as wide as possible for the sand .. one of your local 4x4 shops should be able to recommend the tires best suited to your type of sand .. they may also recommend lift and rims as well. Your 4-banger might not be able to push the bigger tires as easily through the sand. For more power out of the 4-banger .. visit the engine-tech section.
If your hubby is interested in swapping in a larger engine, its the same amount of work to do a V8 as it is to do a 6cyl (inline or V). Check my homepage about my Chevy V8 swap .. V8WrangerIdaho did a Ford 302 into his Jeep .. and many others have also done V8's with rave-reviews about the after-effects
Stereo's .. talk to MusicMaveric in the stereo section .. he seems to know sound systems inside and out ..
We have many very knowledgable people on this board .. posting messages in the right areas will get you the fastest response from everyone .. even debates happen, but that will allow you to work out in your own mind the answers to your questions ..