There's no general category like that...aside from there being tens (hundreds) of thousands of different things within the categories (models, brands, types, etc), how easy or hard or cheap or expensive it is to mod depends on how far you have to mod it to do what you want.
If you haven't defined the job itself, you may want to do that, so you can use that info plus the conditions you have to find out how much power you need to do it with, using things like the ebikes.ca motor / trip simulators. Knowing how much power you need will help you pick something that could be lightly modded to do it, rather than having to replace everything on it to supply that much power.
Then you'd have to look at specific models of specific brands and see what they're like, to see what could be changed within your budget to do the job you want them to do under the conditions you have.
