you don't need to store cryptocurrencies "for" clients because unlike fiat, transferring cryptos is easier, fast and a lot cheaper. and that would be also a lot safer. since you already claim it is to "buy and hold" then you must ask for their addresses and pay them right away. then it is their responsibility to learn how to properly secure their coins.
if i were you i would worry about more important matters such as getting involved with altcoins that can get dumped hard since they are pump and dumps even though they may seem like a big deal!