Considering your age and existing financial obligations, you may want to focus on minimizing your debt. At some point you'll want to retire, and you don't want an additional $1000 per month payment to deal with. If you can score very well on the LSAT you may be able to secure a substantial (or even full) scholarship at Memphis State. I think Memphis' average LSAT is around 155, so a score of 165+ might yield some positive results.
Another option is to attend part time in the evening. This would allow you to continue working, and you wouldn't have to take out loans to pay your mortgage. I believe that Tennessee is one of the few states that has state-accredited law schools, and they probably offer evening options. I know there is one in Nashville, I'm not sure about Memphis.
When I graduate I'll be 51 years old. At this age does rank even matter considering how firms will likely ding me for being an old fart?
Whether or not rank matters depends on your goals. Unless you're willing to pick up and move to another city for law school, and unless your goals include large law firms, I wouldn't worry about rank too much. I think it is possible that larger firms would ding you for being older. They tend to hire young associates fresh from top law schools, it's just the industry culture. At smaller offices, though, I don't think it would matter too much. For those smaller firms (or if you go solo) Memphis State is probably fine.
I don't know what your current profession is, but you may also want to consider the fact that you may experience a drop in salary initially. A new minted, inexperienced lawyer in a secondary market like Memphis, especially at a small firm, isn't likely to command a large starting salary. This is also a factor when you consider whether or not to take out loans.