Wattage is the combination of voltage & amperage, it's power output. 1000w = 1.34hp, 700w = 0.93hp, 300w = 0.40hp... so they definitely do not "max out" at the same speeds, that's a combination of advertising claims & legal disclaimers.
Here's some calculators to help figure out what an ebike's capabilities actually are;
As an example - using a common ebike configuration of "48v 1000w" with just these two figures we can determine it's 1.3hp at a 20 amp draw, which means a 48v 20ah (amp hour) battery will last approximately one hour at maximum throttle, roughly 30mph for 30 miles, pedal assistance not included, flat ground.
I'm not entirely sure I'd recommend a used ebike as it'd be difficult to tell exactly what you'd be getting for the money - the battery is by far the most important aspect of an evehicle & it'd be very difficult to ascertain it's actual capabilities & previous usage/charge cycles. Trickier still, many "factory built" ebikes are far more expensive then simply converting a bicycle with a hub motor & battery pack of equal or even greater capacity...
...& a DIY ebike conversion is about as complicated as strapping the pack to a rack & changing the rear wheel