I built the site using Umbraco, with a separate application with a database of vehicles. I used the same database as Umbraco and prefixed all of my custom application tables with a few letters to make them easy to distinguish (ex: vehicles_xxx)
I had no problems with this agreement, and I do not think that there is a big risk. Of course, you need to take care to update Umbraco (never update the environment live to full testing and, preferably, do it locally), but it is unlikely that the script update will ever change or delete any tables, to know.
There heaps of doco are available for umbraco now - much more than when I started .. however, such a question is always better for forums. :)
all the best greg
Jeeby
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