Put it this way - Spend as much money as you feel comfortable with.
What's the price difference between the Tiagra/105 bike between the one you posted? And hey, nothing wrong with Acera. It's a good, quality groupset. If it hasn't failed on you, then you're fine!
Not that this should matter a whole lot, but keep in mind that Shimano 105 is moving to 11-speed. And I'm sure a lot of the folks here will agree - get fitted. Once you get your fit down, it doesn't matter which groupset you use. If you can haul ass on a hybrid, you can do just the same on a road bike.