Most ebike battery packs do not get too far over a kilowatt because the pack alone is going to weigh in at over 9kg (20 pounds). Once you get to 1000 watt hours, its called a one-kilowatt battery pack, and that’s how electric car packs are measured. 2500 watt hours for an ebike battery is going to weigh in at over 22.5kg (50 pounds) and you're completely leaving ebike territory and moving into electric car territory.
One of the things to look out for is that ebike sellers can sell you on a high Amp-hour pack, but miss the fact that it's actually low voltage. For example a 36 volt 10-Ah pack does not have near the range of a 52V / 10-Ah pack. To do a comparison on ranges of two different battery packs, they must be the same voltage, in which case all you would need is the Amp-hour rating to compare the difference. But…if the two packs are two different voltages then you must calculate Watt-hours. Watt hours are calculated by multiplying the batteries volts by the batteries Amp-hours. A 36 volt 10-Ah pack has 360 watt hours and a 52V 10-Ah pack has 520 watt hours.
Once you know how to calculate watt hours you can fairly well know what you are paying for in an ebike battery pack!
In terms of safety make sure to fit torque arms on each side of the front wheel hub. These will prevent your ebike hub wheel from coming loose or spinning off when under power. Make sure you spend the few dollars extra for torque arms if your kit doesn't already include them.