I think for prototyping that could be fine, though I have usually struggled to get things working when I buy pre-made assets online. Like the scale is wrong, or the materials don't work, or they use super high poly count for no reason, or sometimes the files don't even import. Only maybe once or twice have I bought assets and had them work with no BS.
Also, there can be a stark contrast in styles between artists, even those two examples you gave are wildly different and would probably look strange in the same game. I mean, you could get lucky and find suitable assets but ideally you would make your own art (either yourself, or with freelancers) that would be exactly to your specs.