Regarding heaters, rule of thumb is 2.5 to 5 watts per gallon. So a 150 watt heater would be on the large size for 25 gallons. However, assuming it has a thermostat, it shouldn't overheat your aquarium as it should cut off when the water gets to the preset temperature. The risk would be if the thermostat failed your heater would heat the tank excessively, more quickly than one of a lower wattage.
With regards to can you do water change with just a hose, i suppose yes you can. But i wouldnt advise it. You need to somehow create the syphon, and the only way i can see of doing that with just a hose would be to suck on one end. There is a risk of disease and getting water in your lungs. I dont understand the relevance of the black substrate with regards to this question.
With regards to the gravel cleaner/water change syphon, generally they are the same thing. But, i have seen some gravel vacs that wont do water changes (or at least wont without extra bits of kit).