DX Wombat and others should note that there is no weight limit for a JAR PPL or for the SEP rating included within the licence.
I'm a little puzzled. I don't recall mentioning anything about weight. As far as I am aware the AN2 is the largest SEP in existence which is still flying. Surely that then makes it the largest SEP which can be flown on a PPL(A) subject to differences training?
OF - do it.
I started my PPL just before I retired and it has been the best thing I have done. You won't regret it.