Hi Douglas.
This is a great direction for your business and no doubt the future. Well done for taking the plunge with buying a piece of software to help you grow.
As for your questions:
Key features used ? barcoding, csv, xls, xlsx merge PPMLBarcoding is useful if you are planning on a lot of print storage or direct mail for your customers as they can help shorten the processes invoved in marry items up, collating sets or tracing stored items.
The csv xls and xlxs as im sure you are aware are the spreadsheet files used for the majority of data used in data merging there is software that will render the files on the fly so the user can see the merged fields but most supply you with the files in a 'job bag' format so you can merge using your RIP or in house software. As a note the CSV file is the data in spreadsheet format without any formatting and in its raw form so if you were going to request anything from your customers to make your life easier csv is the way forward.
Are you using two separate systems for online and VDPOnline ordering is commonplace and should be a normal stait of affairs for the dynamic printer be that in a business to business (b2b) or business to consumer (b2c). Depending on your production equipment, what do you offer as a service? Online ordering can be just templated file upload and buying from a shopping cart using a set price. The more advanced softwares will be able to manage stock control and have the ability to manipulated artwork online like moonpig. the VDP elements are just coming to the consumer models now and will be more prevelent in the next 18-24 months. There is software available for you to buy that will run your whole operation including the back office MIS, have a look at http://www.businessprintingforum.co.uk/viewtopic.php?f=16&t=6 for more info. As we found out it really was down to budget!
Features that do not exist that would be welcome in a new productThink practically with this one, we found that although the software did the latest things with artwork etc to manage the jobs out of the system was somtimes a nightmare so i would recommend getting a piece that works as closly as you work at present, this will mean least disruption etc.
As for the online tools keep the customer in mind, the User Interface (UI) is as important and the fancy tricks the software is capable of i.e. if your cusomter finds it hard to navigate around then they will not purchase and you won't get any feedback! they are fickle beasts

I hope that helps.
Regards
Lawson