I must admit I know very little about gray market cars, but I would think the risk of ownership would go down with a used car. Like you said the warranty is expired and I would think selling a 99 in a few years would be less risk of a big loss as opposed to a newer vehicle. From what I have read it is not that uncommon.
Still I would research it as best you can. Here is a link to a forum about importing Canadian cars to the U.S. (You may want to read all 20 pages.)
http://townhall-talk.edmunds.com/[email protected]
Also here is a Canadian car magazine that may have some info on used 4Runner specs???
http://www.canadiandriver.com/index.htm
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2001 & 2004 Toyota 4Runner SR5 4X2
Both V6 and both Natural White
"Yeah, I like white and I like 4Runners"