I decided to rewrite the site code utilizing jQuery and some suitable plug-ins like Datatables. This has taken longer than I expected. Currently it looks like the new site will be ready in 1-1,5 months. I could have re-used the old code adding user management, but jQuery does make it easier to add more user-friendly AJAX features.
Based on some recommendations and testing I decided to switch from Apache to nginx. Nginx should perform better with less resources. Porting the old code to nginx was quite easy. Basically I had to drop Apache rewrite-rules and implement more portable way of using “fancy URLs”.
I might open the new site still with old code and layout in case there will be more delays with AJAX-stuff. Comments are welcome.