* Varian says its offer is superior (Recasts, adds shareholder and analyst comments, background): SYDNEY/HONG KONG (Reuters) - Chinese private equity firm CDH Investments lobbed a last-minute $1.4 billion offer for Australian liver-cancer treatment firm Sirtex Medical (SRX.AX), trumping Varian Medical Systems (VAR.N) days before the U.S. firm was set to seal a takeover deal. CDH’s A$33.60 ($25.33) surprise cash-per-share offer is fully a fifth higher than Varian’s, at A$28 per share, and it lands amid a multibillion dollar shopping spree from Chinese interests in Australia’s healthcare sector.