I suggest you contact PC World and ask them about the exchange value of the vouchers.
i.e. Ae they only for a new computer,Can I use them to buy add-on's( printer,fax,software etc).
As for Future Proofing- There is no such thing-- You can get the best machine for your needs today, and tommorrow a newer macher, better graphics,memory etc may come out.
If you are condidering music production you may find Apple-Mac, have better hardware / software, but it may cost you more.