Postnuke Upgrade

Hi, I am the owner of [login to view URL] with CMS postnuke. Our site has been hacked many times as we didnt fix the security problems i guess. I want it to be upgraded to latest version and security problems fixed. Site has been built like 1.5 half years ago. I have little knowledge about postnuke ana the guy who built the site cant help me anymore. I fixed the site with back up after it has been hacked. I dont out the [login to view URL] as i am afraid of a new hack. What will it cost to upgrade it? And i may ask a xanthia template too. But i wish to have it hackproof. Details About the site 1)Version is i guess 0.761. Site was built 1.5 years with the latest version of that time 2) We uploaded many modules but the ones we regularly use are Permissions, pncommerce, PNphpBB2, Search, sections, Submit_news, Top_List, Topics, Type List and pnlashgames. I guess there is at least a block for pnflashgames not sure about the rest. 3) We have 7.000 users. Best regards Selim Gurcan