I am not familiar with DW Appliances, but you should follow the same procedures as for any other acquisition. List features you must have, would like to have, nice to have. These requirements should come from the technical group but based on both business and technical requirements.
Send out an RFI to various vendors asking them to comment on your requirements. After getting responses, revise the list of features and assign weighting factors to them.
Grade each vendor on their features. Decide on a short list of 2 -3 vendors.
Evaluate the vendors based on other factors such as industry standing (Gartner, repuation, etc). Evaluate their vision and strategy. Call their support and help lines to evaluate their support. Check their online support. Evaluate third-party support and plug-in products. Check the financial health of the vendor.
If the answer is not clear at this point, it's time to consider prototyping or proof of concept with no more than 2 vendors (too many vendors are difficult to manage).
Please feel free to provide your feedback and results in this forum.
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The trouble with doing something right the first time is that nobody appreciates how difficult it was - Steven Wright