Mac OS X does not have TRIM support, so you'll be limited on the "reconditioning" part of a solid state drive unless you buy a SSD that has error handling. I personally use OWC Mercury Pro SSDs because of their reliability and warranty. Plus, they sell the best solid state drives in the industry.
I wouldn't recommend using a Mini for web programming since PHP and MySQL will most likely be serving your back-end. I'd recommend a simple Linux box (Debian) over any Mac OS X or Windows configuration. Place lighttpd, PHP and MySQL on the machine, and you'll have a machine better than any Mac can support.
Don't get the SSD unless you absolutely need it. Needing a solid state drive all depends on how much load the Mini server is getting. I still use 7.2k RPM drives to handle 580k+ monthly active users and my server handles it just fine. If you're thinking to develop large scale applications, I'd look more into adding more memory in over solid state drives.
If I'm correct, the Mini has a Firewire 800 port which has a data rating of 800Mbps. Internal drives run at SATAII, 3.0Gbps. If you're planning on using the external drive to serve content, I would not recommend it.
If you need expansion and robustness, I'd look into building your own Linux box. Not only is it cheaper, but it is more flexible to expansion and upgrades in the future. Also, if you're new to web programming to the masses, look into source version control too.