Is there anything in scipture or tradition that requires belief in the Trinity for salvatiion? I can say in scripture no. What says tradition and who wrote it?
As for needing the Holy Spirit in an individuals life....this is far better than I could ever write:
Using the bible, the Spirit teaches and convicts us of the truth and assures us that they are saved (1Jn 3:24, 2Cor 1:22, Eph 1:13; 4:30)
It is the Spirit in our lives that unites us to Christ in his death and resurrection (Rom 6:1-11, Gal 2:20). Being ‘in Christ’ by the Spirit secures our completeness (Col 2:9-10). Being ‘in Christ’ by the Spirit unites us with others (1 Cor 12:12-13, Eph 4:4-6)
He empowers us to do what is right, live in faith and do good works (Ro 7:18, Eph 2:4-10). It is always God’s work in us that means that we are saved in Christ and living new lives for him. It is the Spirit that makes Christians aware of their sin and empowers them to turn away in repentance.
It is the Spirit who distinguishes us from unbelievers (Eph 2:12,1 Cor 15:21ff) and empowers us for mission in the world (Acts 2:1).
The Spirit is an essential part of the Christian life, the Spirit is how God works in his people until the second coming of Christ. The Spirit helps Christians to understand the word of God and live it out in their lives today.
http://christianity.net.au/questions/why_do_we_need_the_holy_spirit