Apart from "How did he buy that much with $1,000,000?", there's also "Where do you find an abandoned castle in the United States?" This was Lampshaded by Shay.
...Texas, it turns out. 1415 Wooldridge Drive in Austin, to be specific.
Why did an FBI agent let Preston, a juvenile delinquent who inadvertently laundered $1,000,000, not only get away, but kiss him in front of her coworkers?
Aside from the dubiousness of putting his own values on the check, possible forgery concerning the house, and not getting adults involved sooner, he didn't actually commit any crimes. He got the money legitimately from a legitimate source he had no way of knowing was crooked, and spent it on legitimate items. As for the agent, being undercover at the bank she knew from the beginning that he had gotten the money and needed to prove it, but apparently not keep him from spending it. At the end she thought he was a sweet kid and left him hoping rather than crushing his crush- the kiss was a reward for his help in the investigation, and that's how she'll explain it to her coworkers. Finally, it's a 12 year old boy wish fulfillment movie.
A synthesized voice has a peculiar quality most real human voices don't. Why wasn't this picked up by the sellers of real estate?
No reason for them to care, a buyer is a buyer, and a check is a check. If some guy is a security paranoiac that only speaks through a computer, who are they to judge?
Maybe they assumed that Mr Macintosh has a vocal disability and uses a machine to speak for him?
Preston's home gets broken into by the crooks at one point. They even break the door down. And then they leave. This never gets brought up again, even when Preston is brought back home at the end of the movie.
Why would the brothers start a computer startup despite not knowing how to use them? And their father prefers them to Preston to the point of letting them take over room? In fact, why the hell does he berate Preston for not starting his own business even though he's (in the immortal words of the Nostalgia Critic) like seven, ie clearly too early to do so anyway?! Other than him being a Jerkass.