I would advise to get a vendor that specialises or have previous experience in doing what you want i.e. redevelopment of online catalogue website. Having a look through some vendors portfolio's and asking specific questions will help filter out the inexperienced contractors from the experienced service providers. When you've sourced some of these providers, then filter them according to quality, price, timeframe for delivery and after sales service and support.
This can easily be done by contacting some relevant vendors or else specing the project out putting it up for tender on one of the sites outlined..... absolutely nothing to lose... depending on how well you've detailed your requirements, it could very well work out cheaper and of an extremely high quality. Even if you put up your tender and decide not to proceed, you will have received lots of helpful information and alternative viewpoints from prospective interested parties.