Ha. Looks are totally opinion. As for the aftermarket; there are tons of products available for all years. If you mean the supercharger; I thought it was available for '03 and if not, you are talking an extreme minority .01 of 1% would even consider it.
What you get with a older model year is cheaper. The mileage is pretty good for that age and assuming it is in good condition, may be a nice deal.
So to your issues:
1. Seat wobble. Yea, this is a notorious issue. Mine has it as well. It changes based on where you set your back rest at. I adjusted my back rest till it went away. It is probably a design issue.
2. Timing belt. Look up the truck at the toyota website. Input the VIN and it will pull the records. Second, look for a white sticker on the radiator. If the belt has been replaced, you would see a sticker on the radiator with the date and mileage.
If you can't find evidence it was done, then assume it hasn't and have it done once you buy it. Figure 1k for negotiation purposes. Whatever price they ask, expect at least that much off.
So. A '03 with 126. I think 8 or 9 is a valid price.