Just looking at the CL ad it looks like it has newer BFG AT's which would cost a good chunck of change to put on there. So thats definitely a positive, cause if it needed tires thats a good $600-$800 right out of the box.
As far as the engine running rough as the speed increases, I would have a diagnostic ran on it and see what codes are being thrown before I bought it, it could be as simple as plugs and wires or a vacuum leak somewhere. On the 87 trooper I had...about 14 years ago it kept blowing a spark plug. I would put a new one in it and about 5 miles later it would blow, and I could always feel when it happened cause of the misfire and loss of power. This was a 4cyl though not the v6. Anywho I ended up dumping about $1400 to fix the issue...I cant remember exactly what it needed as it was over 14years ago and I wasnt big into fixing my truck myself just beating the ******* out of it lol.
I noticed that most of the carpet that been pulled, that kinda makes me wonder if the p.o. has had it completely submerged in water and pulled the carpet out after, if so the engine issue could be related.
On a side note I would buy some rolls of industrial rubber mats and custom fit it to floor, that would look sweet and not to expensive also bery functional. Or even cheaper I found large cargo rubber mats at sam's for $20. I thought about buying a few of them and cutting them to custom fit my rig.
I think the highest I would go on this thing would be $1200, because it has at least $700 worth of newer tires.