The others have offered good board advice, so I will comment on your 50+, 80kg, back pain issue.
I have been sailing & racing Formula for over 6 years and a couple of years ago had serious back pain too. Went to the doc and found out that I lacked core strength, so I went to a physical therapist to get started on a strength program. I now work out three days a week to build/maintain overall strength including my core as well as stretching on a regular basis. At our age, If you plan to stay in the sport, you have to have a regular weight program. Make sure you see a Dr. and find out exactly what is happening with your back. If you have a disc problem, weights may not be a good idea.
All this resulted in a huge improvement and has almost elimnated my back pain. I too weigh 80kg and am 62. I have no problems sailing with an 11.0 on my formula board in conditions up to 12-14 knots, but I change down to my 9.2 or 8.4 pretty quick as the wind picks up.
Yes - downwind on a formula board can be scarry, but that's what makes it so much fun. Push yourself and take tiny steps and before long, you will be shocked at what you can do.