A properly broken in 650 can give years of faithful service. You may not have as much UMPHH on the top end as a 750, but you will still have a good bike as long as you realize that it is not meant to get out on the superslab and go like the dickens all day long.
Spare parts for the full time 2wd are getting hard to come by, but if it blows up on you, I believe you can change everything out for a selectable 2wd system. You may want to look up the phone numbers for Holopaw Gene, Flicka at Wagner's or Terry Crawford and talk to them about it if it worries you.
If the bike is in good running condition, and you think you are getting a good price for it, I'd say buy it and use some of the money you would have spent for a new(er) bike for a possible upgrade to a 750 in the future.