If you buy one then mod it, it just ends up like triggers brush. Whether you buy one or build one, if you intend to diy it afterwards you only miss step one.
You end up building anyway.
But it's easier starting from the ground up than buying one, you hit a problem with an esc. It could be any number of reasons. Like motor or fcb. If a motor ain't working someone without the know how may just buy another motor & that doesn't work.
Unless you got money to burn & you want to get into this hobby. Learn to build, it's easy. It saves money & time.
And this is assuming we all want brushless setups or whoops & none of them nasty VL toys which can be modded. I been there But who wants the fiat 500 when you can have the veyron.
& be your own mechanic.