Welcome to the Lexus Owners Club Peter.
I just went over and looked at the Kelley Blue Book to find the current value of a 2005, GX with 100,000 miles. The current value for a standard equiped GX470 is $16,883. This is the retail value, not the trade in value. It also reflects the value of a car in what is judged to be "Very Good" condition. If it is just "good" condition it will be worth less.
I can't say wether this is a good buy or not. That's up to you. But, a couple of things. You say all taxes are included. I don't know about Ontario but that price with taxes included would be worth a lot here in Iowa. Mileage of 103000 is not much to worry about. Many Lexus run well up to near 200,000. As with any well enginered luxury car, specific, regular maintenance are always important. The GX has a great engine.
No one can tell you how much it cost to maintain. It all depends on how you drive, how many miles, climate, road dust, highway miles versus city, etc. You'll want to read your owners manual front to back and you'll find the specs, for the GX maintenance.
You'll want to talk with your dealer about which oil you want to use. Or, go to the top of the page to the right and do a search about oil types for your GX.
Enjoy!
Paul