Thanks for all the info, folks.
I just completed OW in a scuba park near Terrell, Tx. The water temp was 47F. I wore a 7mm farmer john with a 7mm step-in jacket over it. Also wore 5mm hood and gloves, plus 32 pounds of lead strapped around my waist. Ahhh...good times...good times...NOT!!!
As mentioned above, I was looking for that "one magic suit". Not going to happen. In an 80F pool, I used a 3mm shorty and started to get chilly only when I surfaced an hour later.
Water temps in Texas lakes for spring-summer-fall tend to be around 70. Therefore, from the above, it appears a 7mm full wetsuit is the starting point. When I head south of the border (Cozumel, Cancun), I'll invest in a shorty (a friend of mine dives sans wetsuit there).
If I dive with a 7mm in warmer waters (80F), would I be too hot underwater? Or, is that just a dumb question? I'm ignoring the comfort and flexibility issue here.
I should have mentioned my physique earlier, as that has an impact: 175 pounds, 5'11", 55 years old. Yes, I'm just starting this sport at 55; better late than never.