The DS214 should be able to handle 2-3 clients for MySQL. It really depends on the size of the library and memory. Synology claims the DS214 can host 30 virtual websites (I wouldn't like to see that). If you want more memory go up to the DS214+ or DS214play. I currently have an auction on eBay (same ID as here on AVS), selling my less than 60 day old DS214+. It started as a Survelliance Station box and DSM 5.0 seemed really fast. 3 720P HD recordings consumed 10-18% of the CPU and less that 15% of the memory. I also used it as a network backup for documents only from my primary DS1813+. It was a purchase decision to try to save costs. I ended up going with a second Intel Atom based DS1813+. Mostly, because I wanted to backup everything, not just documents. The added advantage of the same model means I can use SHA.
I digress. My recommendation, if it's in your budget is to get an Intel Atom based NAS. They can do so much more (i.e. run Plex, packages for x86, etc.). The DS713+ is a 2-bay model and performs very well against all other 2-bay Synology models. Do not let the model year drive your decision (learn from my lesson above).