The 7.5 VDC may (and may not, but we don't know) be important, so you'd best stick with something very close to that.
The 1500mA is a minimum. Which means that the unit you describe below as 1700mA is just fine. The only catch with the current is that if it says some specific voltage at a specific current, as the actual current drawn goes down the actual voltage delivered will go up. Hence you don't want to use something able to supply twice the required current simply because it will be so under loaded that the voltage will be significantly higher than what it would be with a full load.
However, as noted, the voltage doesn't always make any real difference either! Lots of units are powered with "switching" power supplies, and the voltage chosen is merely a convenient one. "Convenient" may be the best deal they can get on purchasing bulk orders, or may be related to a size that is commonly available.
One example that has been discussed in detail in this newsgroup is the power supply for Linksys WRT54G routers. They come with a 12 VDC 1A supply. The unit will work off a supply of less than 5 volts to more than 20 volts though. The actual power used stays about the same, so as the voltage goes down the current goes up. Using a 6 VDC supply it would be a good idea to have one rated at 2 Amps. An 18 VDC supply would probably do fine if rated at 750 mA.
As Jeff Liebermann originally pointed out, with the WRT54G the upper voltage is probably limited by when the capacitors blow up (literally).
The 1700 mA unit is perfect.
The 1000 mA unit would almost certainly work. The question merely how long before it failed. Not if, just how long. At that difference in rated current, it might fail within minutes. It might take months. You'll have the 1700 mA one stuffed in a box on a shelf, just in case, long after the device it powers has been tossed into the trash.