Just a starting guideline, try 38 front, 44 rear. The pressure will increase as the tires heat up.
If you are running SCCA autocross, a strut brace will move out of Stock class, unless it is a BMW brace that was a factory option.
If this is your first Autocross, a strut brace is a waste of money. You'll need to run at 10+ events before the strut brace will make any difference in your time in terms of helping to make you more competitive. Spending $100s or $1000s in mods won't help if you don't understand how to drive. Rule number one: "In order to be fast, you have to slow down."
This is the order of "tuning" for autocross:
1. Driver skills
2. Driver skills
3. Driver skills
4. Car general tune up
5. Tire upgrade
6. Suspension upgrade