Do I bear sin if I reject a Christian young man who wishes to embrace Islam on the condition of marrying me, and is this considered depriving him of guidance?

The sister incurs no sin by refusing the unbelieving young man, even if her refusal prevents him from embracing Islam, because she is not obligated to accept him. What is obligatory upon her is to sever her relationship with him, and she can provide his address to da‘wah (Islamic propagation) organizations. It should be noted that if his intention to embrace Islam were sincere, he would not have made it contingent on marrying her.