You are on train 785, which is one of the busiest Surfliner trains. However, you are getting on at the second station, so you should have no problems finding a seat. Plan to have a seatmate by the time you get to Solana Beach or Oceanside.
Some prefer the business class upgrade and others don't think it's worth it. The seating is very similar to unreserved coach but you get a complimentary newspaper, snack, and drink. However, with business class you a guaranteed a seat. I purchase the upgrade only if I expect coach to be standing room only.
I believe Amtrak's OTP is based just on train's arrival at the endpoint, which for 785 is Goleta. Typically the Surfliner trains start on time and if they fall behind schedule, the delays are encountered enroute. Your train will most likely be close to on time at Old Town.