Temperature: As altitude increases, temperature decreases, by about 2 degrees C per 1000 feet above sea level. Temperatures will dip below freezing at around 7,500 feet up, so you'd probably want to stay quite a bit below that or risk frostbite. Especially with windchill, any exposed skin will be at risk. This temperature change occurs relative to ambient temperature at the ground, so you could go a bit higher relative to sea level if you're in a very warm area, or if you're in an area with a high elevation, which will be somewhat warmer than just air at the same elevation.
Oxygen: The amount of available oxygen decreases with altitude. The FAA requires pilots to use supplemental oxygen at cabin pressures above 12,500 feet for 30 minutes, on the assumption that the plane will immediately dive to an altitude with breathable air pressures upon loss of pressure, and at all times at cabin pressures above 15,000 feet. You begin experiencing notable reductions in performance from hypoxia above 11,400 feet, though the time of useful consciousness remains indefinite until 15,000 feet, beyond which it begins rapidly declining.
G-force: This is mostly a non-issue - so long as you're not speeding up, stopping, or turning too quickly, you can keep your G-forces at manageable levels. The biggest limitation will be your turning radius at high speeds.
Top speed: Since these people can accelerate without bound, their maximum speed is theoretically unlimited. Once breaking the sound barrier, however, there will be significant heating due to air resistance, as pressure increases dramatically once you are flying into your own shockwave. According to this question, speeds in the range of a few thousand km/h could cause heating in the range of hundreds of degrees C. Air density decreases with altitude, however, so you will be able to go faster at higher altitudes. At any rate, the sonic boom when breaking the sound barrier will also be quite unpleasant, so you probably want to keep it under Mach 1 (1234 km/h). I'll also mention that even windspeeds in the low hundreds of km/h can be very damaging, uprooting trees and leveling homes. Unless you're wearing specialized skintight clothing, whatever you're wearing will likely get ripped to shreds if you're flying faster than perhaps 300 km/h - I hope you don't mind arriving naked.
Overall, humans without any special equipment might be limited to flying at sub-sonic speeds under 1000km/h, and may only be able to fly at a couple of hundred km/h comfortably. Top altitudes will be about 5000 feet, depending on local climate and geography. With warm clothes and flying at low speeds, one could perhaps get up to 15,000 feet in altitude before availability of oxygen starts to become a big issue. Otherwise, the limiting factors will be air resistance for speed, and temperature for altitude. Since these work oppositely (altitude makes you too cold, speed makes you too hot), you might be able to balance these somewhat to go faster at higher altitudes, but you'd probably need to be spinning constantly like a rotisserie chicken (since only your front side is being heated) and you'd risk burns or frostbite if the temperature balance didn't work out just right.
