If you want to buy STEEM for investment purpsoses, of course you can. But there is no need to buy if your purpose is to upvote somebody. Keep on posting so you can earn Steem and SBD and then convert them to Steem Power (SP) so that the value of your account goes up and at the same time your reputation goes up too. After 7 days, you can claim your reward as Steem and SBD (50%/50%). The upvote you are giving to someone´s post is being paid by Steemit´s reward pool and not deducted from your account.