I would buy a used 5hp engine of any make before the Briggs, and I live about 20 blocks from Briggs headquarters. It would cost the same and be better in every way, except if air cooling would be an advantage to you (it would be a disadvantage to me).
Most 5hp engines will have liquid cooling, larger range of speeds, have two cylinders for smoother operation, variable ignition timing, underwater exhaust..... Since the early 80's their HP is rated at the prop, not the crank, and any 5hp outboard will certainly outperform the B&S lawnmower engine.
That's not to say it's a bad product. If you're unwilling to buy used, it's probably a good option especially if it gets used seldomly.
As to the idea that the B&S is more reliable than a comparable outboard... are you kidding?