Firstly you mentioned you do plan to go off road.
If you plan to go through water, big exhausts are not so great (depends how deep you are willing to go into water), and there is not enough exhaust pressure to keep the water out. So keep that in mind.
Personally, i would keep the same size, and just look at high flow with that in mind.
Cats - universal, just high flow. Come in all shapes and sizes.
If you go dual pipes, and they are both 1/2" larger than the current single pipe, that is a huge difference.
Dual in - dual out - saves space more than anything. Depending on the brand and the design, would depend on the sound it produces.
In terms of sound your after - talk to a exhaust specialist in your area, and listen to what they say. The will most likely recommend a product or two, and its hard to say what it will sounds like, but i guess you tube around for that.
I would also talk to a 4wd specialist (the exhaust guy may know?) about the size side of things and in terms of back pressure and water.
I had to cross a big puddle of water in my last car - BMW 740 4.4, with bigger exhaust and full high flow.
I kept on the brakes, and kept the foot down to produce as much back pressure as possible while going through slowly. (btw the tips of the exhaust were hidden, and pointed down) As the exhaust was bigger, not enough back pressure water got into the whole exhaust and the car was sputtering until all the water evaporated out the exhaust. So i would b every weary and keep in mind where you plan to take the car. For me its more about practicality than looks/sounds - but that will be a balance that you can decide and your plans for the car.