There are no "bad" schools that I'm aware of in Scotland so you'll be fine wherever you choose to fly, Edinburgh and Glasgow are more expensive and they will also have some limitations in training due to being at the busy airports. Cumbernauld is likely more expensive than Fife (Glenrothes) or Perth but you should take in to account the travel costs of getting there
. Personally having lived in Glasgow and Edinburgh and learned to fly in Glenrothes, I would go to Cumbernauld it's closer
and will be a fair bit cheaper than Glasgow/Edin
As for how long, it will take you between 45 hrs to 55 hrs I reckon for a chap of your years. You can expect it to take longer than that if you do a few lessons and then take a long break, how much longer will depend on how long a break you have between flights. As for time, well as you say that depends on you, the fastest I've known was 3 weeks, the longest was several years without a lengthy break. You also need to do the exams, they do an intensive groundschool in Inverness that I know is excellent and you can do your PPL up there as well, they are also very good indeed
so worth considering if you fancy doing it intensive. I know a few guys in your work situation that have done it this way with significant success.
Weight wise, it's hard to say without knowing your situation, if from a BMI point of view you're overweight then you'll likely be fine, if you're obese then that may be an issue. The limitations of the PPL medical are on the CAA website if I remember rightly but you should contact an Aviation Medical Examiner, they'll talk you through it and will arrange a Class 2 medical to find out more.