Happy 10yr anniversary (site lists you as member since 03/2010).
On to the M-series. As the owner of two M35's let me say ... fear not. There is, of course a caveat ... The 3rd and 4th gen M's (2006-2008 and 2009-2010) are pretty much bullet poof .... IF ... they have been maintained properly. Mine were purchased with relatively low mileage so no major maintenance was needed by the previous owners.
I can't tell you that yours will be as reliable unless you can see the maintenance records regarding fluid changes, plugs, belts, hoses, etc. You can take it to any Infiniti dealer for an oil / filter change and get a pretty comprehensive inspection included. That should give you a starting point. Maintenance on any mid to high level car is pricey and M35's are no exception but you can budget them in one by one and you may be able to do them yourself. There are DIY thread in the FAQ section of the site.
I can't speak on the M35X models because I'm, in South Florida with no need for AWD. I can tell you that over the years I have not seen members complain about the AWD system very often.
My 07 M35 had the rare bourbon leather and was assassinated at just over 100k miles by a drunk 21 year old tart. Infiniti dropped bourbon leather in 2009 (just when other car makers were adding it and selling them like brownies). The 07 repairs were limited to a starter, 02 sensors, and hood struts over the entire life that I owned it.
My 09 also has an option that is rare called the 'premium package'. It was a $9,000 option which included all the xtra packages and added temp controlled, reclining rear seats with a movie screen that lowers from the roof. That was the last year infiniti offered that option. It now has over 110k miles. The only repairs to date were the catalytic converters and the air/con evaporator. I also had the dash replaced due to the air bag recall but that was not a failure - it was cosmetic.
Mind you, I spent a lot of money on maintenance. If you cheap out on the maintenance, your bills will be likely be higher.