It really depends on what you mean by 'expedition rig' (BTW, you might check on expeditionportal.com, but there the LCs rule the roost). If you want a car-camper rig (that is, don't really plan to travel the Amazon Basin for months at a time) the LC is probably overkill. It does have room for goodies, will be the most rugged (I worked on a friend's LX450 recently and the rear end on that thing is like a 1 ton truck rear), and probably the most capable (again, depending on what you plan to do with it).
However, that LC is pretty expensive for that year and miles. I found a 96 LX450 for my friend with 196K on it for $3,300. He put another $700 in it for maintenance (repack front bearings, new A/C compressor, various other minor things), and $1000 for an OME lift. He wants an ARB bumper and winch, and will go with Cooper AT3 tires. Those additions will add about $2500 to it. So his total investment will be about $8000 and it will be about as good as the one for $13,500. He won't have lockers, but adding ARB air lockers would still have him at just over $10,000. It was also driven by a lady school teacher so it is pretty much an off-road virgin (built trucks usually have had an 'exciting' life and I try to avoid them). If you want to do the build, there are better deals out there.
The 4th gen would be a good option, with a little more room than the 3rd gen, and of course, newer. Those prices are reasonable, but I've seen them for less. I looked at an 06 GX470 a while back with about 90,000 that was perfect for about $13,000 at a dealer. Of the ones you linked to, the 04 is not a bad buy, but again, it's probably had an exciting life. If you want a real 'expedition' or overlanding rig, the key feature you want is reliability, and a truck that has had an exciting life, or with too many hard-core rock-crawling doodads, will likely suffer in that department. If the 04 has what you want on it, and you're not afraid if its history, you couldn't build one for that price with those mods.
If you just want to be self-contained and be able to sleep, and your current rig will go where you want and do what you want, the RTT is the cheapest option. You already know it, and it's going to get MUCH better mileage than the LC (think 12 MPG around town and a little more on the highway, which will add $$$ quickly if it's a DD). I have the itch for an LC about every 3 weeks, but since I drive my 3rd gen everywhere, the gas $$$ would break my budget (the 3rd gen is bad enough). And unless you are going out west to do your 'expeditioning', the 3rd gen will go places the LC won't (in the midwest, where I live, many of the Forest Service roads I run are too narrow and brushy for an LC to get down without serious damage, and I've been on some an LC just would not get down at all because of the size). And there's no vehicle cheaper than the one you already own. But, again, it really depends on what you mean by 'expedition rig'.
