Tim
If you are looking to pack lite skip long johns. I never wear those even in the dead of winter on the course...it will September
No need to pack loads of balls either. Bring what you can fit in the bag and if you need more buy some.
Don't be reluctant to stay at a great course for the day if more than 18 is your bag.
Again, if packing lite, yes, wear shorts, especially if it rains. Why get your trousers wet and mucky...save them for evenings.
Don't buy an expensive gortex rain suit simply because you are going to Scotland. A wool jumper, inexpensive bottoms, rainproof short sleeve top, an umbrella (which can be bought at the club if need be) and MacWets will do the job 99% of the time. For the other 1%...wait the weather out...its not like you have a train to catch.
Many swear by 2 pair of golf shoes, using the travel lite mantra...if I fly its with one pair. If you are into the new style Ecco shoes etc, tons of clubs now sell them and with the exchange rate as it is, if you find yourself desperate (for some strange reason which must have to do with bad feet) for another pair...buy them.
Play the Himalayas with everybody in tow...trust me, it will be enjoyed. Also, take a good look around the nooks and crannies of St Andrews with family in tow.
Ciao