Welcome to LOC team. :)
First off, congrats to you & your Wife on expecting your first child! B)
What is your price range for a vehicle & just curious what is it about the RX specifically you like? Do you have any other vehicles on your short list other than the RX & possible Highlander?
As you seem to know, the key with one of these vehicles is service records, service records & service records (especially all oil & tranny fluid changes accounted for & I would only consider one if the timing belt has been changed as well). My main concern with any 8 - 10 year old vehicle is the lack of a warranty...oh sure you can get '3rd party' coverage...but on a Lexus....this will be expensive & read the fine print very carefully. But definately have a trusted mechanic go over the entire vehicle with a fine tooth comb before any deal is done.