"While the Pi is limited in processing power and I/O" This line is the key to the question. Simply roll back time until the specs of the Pi would be a "decent

"While the Pi is limited in processing power and I/O" This line is the key to the question. Simply roll back time until the specs of the Pi would be a "decent" or at least "cheap" webserver for that time, the kind of server you'd run a small forum upon at that time. OR if not decent for bare metal hardware, decent for a virtualized image on a bigger server. Its not like 2014 is the first year in human history with webservers or web based forums. So people were running forums on tiny virtualized servers with about those specs on linode perhaps just a couple years ago. Perhaps even today either on very tiny linodes or competitors. The only real problem is the software. So use old stuff open to individual and class of vulnerabilities that were fixed 5 years ago, or take an obese beast of a modern forum and give it a liposuction? Look into puppet and run the master somewhere else and automate it enough that you can go from a bare metal Pi to a live host in ten minutes or so. You'll be doing it again sooner or later at a time not of your choosing, so may as well get it right now. Also, implement a backup strategy and automate its restoral process (and integrate with above)