I’ve always worked as an in-house VAT advisor in the public sector, so I don’t know what the going rates would be for an advisory firm! If you are considering getting paid advice, you could always try a second firm – this may give an indication as to how reasonable the figure you’ve been quoted is.
Generally speaking – when HMRC find errors, they will assess for any VAT underdeclared plus interest. They will also consider charging a penalty, however this can be significantly reduced or even waived/suspended altogether if they believe that you took “reasonable care” to determine the correct VAT treatment, cooperated during their enquiries and (if appropriate) have taken steps to avoid similar errors in the future. Starting a forum thread is certainly an indication of taking care, but it’s possible that going to an advisory firm if you’re still uncertain might be considered better.
Many paid advisors (but not all, so worth checking) are likely to have some form of professional indemnity insurance – it may even be a requirement of any professional bodies that they’re a member of. But I’m not sure whether that would actually cover the VAT itself, as the only real “loss” through following incorrect advice would be any penalty that HMRC may charge.