Most likely the coins in the top 100 ranking in Coinmarketcap website,you can't go wrong with any of them but just to be sure you can narrow down to top 10 to top 30 and you will win quite a lot of money if you start mining right away.In this scenario we don't care that you are mining at a loss as you are in for the next bull run so you can turn on those GPU-s no matter if they are well over 5 years old,in the end this type of experiment I am confident will pay off much better so as long as you have money to pay for electricity even from your own pocket like I did in 2018,you will get rewarded.I did the same in 2018 and in 2021,February Bitcoin at 60.000 I was a happy man.
So this is almost similar to him just buying the coins he has eyes off an exchange with the DCA strategy using the funds that would be spent on the electricity bills to keep the GPUs running except that the GPUs won't be lying to a waste if he just decided to mine instead of buying 
mostly all coins goes up if bullrun comes again no matter what coin you mine.