DreamHost has very strict rules regarding unsolicited email, but, since your emails will be sent to your forum members, that should not be a problem for you.
However, DreamHost also has an email limit of 100 recipients per hour, this could cause you problems with the number of members you mention.
One solution would be to use the 'Announce List' functionality offered by DreamHost to inform your users of updates etc. These lists are not subject to the 100 recipients per hour restriction.
DreamHost also has very strict rules regarding copyright infringement, but if you keep track of what your users are uploading and remove any suspect material quickly you should not have any problems.
I think DreamHost is likely to notify you of any copyright infringement and give you sufficient time to remove the offending item(s) before taking your site down, but it would be best to check with DreamHost themselves for the authoritative answer to this question.
I've found DreamHost to be very upfront about any costs. I have never had a charge from DreamHost that I wasn't expecting.
The 97 day money back guarantee only applies if you pay by credit card. Also, the initial registration for your 'free' domain ($9.95) is non-refundable.
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