let me first address the parts/service area. It will be noticable cheaper at a toyota dealership, for practically the same parts since the two brands use the same engine components etc. However, you will not get loaner cars at a toyota dealership, nor will you get that famous lexus service. If those things arent important to you, then you could still bring your lexus into a toyota dealership and have it serviced there instead. Anything goes since the two brands are interchangable.
I would rule out the LS and the GS seeing how they would probably be more "stripped" then an ES or Avalon in that price range. Also, as i was point out to earlier this year, the ES is actually larger than the GS, so seeing how you need all 5 seats.
LS- an LS in the mid $25K might not be in the best of condition. this would be the old styling and keep in mind (maybe im just nit picking), the 2002-3s did not have the refresh which included LED tail lights etc.
GS- i never like the old generation dash nor exterior, but perhaps you do. ALso this would be the old styling and it seems pretty outdated if you ask me.
ES- an ES330 shouldnt run you more than 26K and it would be fairly new. that styling is a little more sleek than the equivalent year of a GS. The car is bigger as well, and it has 230 hp i think
Avalon- I would probably choose this car, though it would probably be ranked next to the ES. Im one to want kinda a "bling" car with the lexus name on it, but if you dont care, the Avalon would be the best bang for the buck. Keep searching around, and dont rush into it. Also keep in mind goodies like the Nav are the same as the ones found on a lexus. the interface is the same.