Allow me to part some of my experience on you. Others may have some other opinions but this is just what I have observed over the years...
You will absolutely get the best reliability and performance from a stock block below 400whp. The stock block is more than capable of handling that power. No built block will come close to the reliability of the stock block. So in your situation you might be better off getting a crate motor from Subaru. The short block can be had for around $1600 to $1900. That and a good tune and frequent oil changes will get you a long way to reliability
A built block is going to have the forged pistons and rods. The piston to wall clearance are much looser than stock tolerances and that, alone by itself, is not a big deal. However when you consider that you now have very different oil requirements for the pistons versus the main and rod bearings (assuming they are using stock tolerance bearings) and the turbo and head components, you risk compromising the integrity of the motor for reliability and longevity. The key to properly building an engine is to match the clearance globally for what ever oil you are going to use. If you are replacing the pistons with forged units, then they require a different oil to be spec'd. You now have to build the rest of the motor to accommodate that oil by loosening the tolerances all around. Otherwise you risk spinning bearings or galling tappets. The other thing is since the piston to wall clearance is looser, you will likely see more blow by and more leak down. This, IMO, does not make for a reliable motor.
So the moral is, Just because a motor is built to handle 500whp does not make it a reliable motor and, in fact, built motors are typically only reliable when they are used for their intended purpose but become unreliable when used for daily driving use. Most race and performance engines are torn down and rebuilt after every season and the typical user will not be doing that.
Essentially they just want to sell you their engine and get your money, which I don't have a problem with. After all they have to put food on their tables. However I think a little more research is required on your part and a little insistence that they do right by you. IMO it is overkill for your goals and it will not serve you in the end.
Point...
My previous STi went through motor after motor and it sucked me dry of over $100,000 in its 6 year life. I was using race built blocks and was rarely tuned for more than 450whp.
My current car has a stock block with the stock blow by system and has over 75000 miles on it currently. It has 145psi of compression in all four cylinders and the the oil pressure is above 45psi on a hot motor at idle. It was tuned to 430whp for much of it's life. I recently de-tuned it to 368whp. I attribute this to a good tune and frequent oil changes. This car sees both daily driving and hard use.
