I haven't trade any coins there yet, but my friend said you must have at least 0.05 - 0.1 eth. You must prepare enough gas to buy any coins on Uniswap, mate.
I think 0.05 - 0.1 eth is not too much to provide, because it could be the effect of gwei costs from the erc20 platform which is still large and unstable until now, so uniswap users have to provide 0.05 - 0.1 eth to buy coins there.