Hi Steve,
The weather today in Chichester is PERFIK!
Weather minima for training is very difficult to advise on, as it depends on what needs to be achieved. For instance a cloud base of say 1500' or less may be fine for circuit training, TO's and landings, but rubbish for cross country. Wind is again a bit awkward, as down the strip direction it can be stronger than a crosswind. I suppose (happy to be corrected) a reasonable student PPL guide might be? Wind strength max 12-15 knots unless straight down the strip and not gusting. No precipetation and a cloud base of at least 1500'. If the cloud is broken and clear higher up, the instructor would likely be happy to take you up above the cloud for upper air work.
I gained my PPL before human performance came along, so can't really help other than to say that the PPL questions are multi choice and fairly easy if you've read through what's needed a few times and have a basic understanding.
As for young and keen, or old and wisened instructors? I prefer the latter, but if you have any instructor that actually enjoys teaching, you won't go wrong. The older ones "tend" to be more reliable in this respect as they have shown by staying the course tht they like instructing, or have returned having had a long and industrious career with the airlines. Some even combine the two! .... Dodgy subject though, so put on those flameproof trousers!
SS