How many datasets types do you need to access and do you need to access them live (i.e. 1 oracle database or 2000 seperate GIS and CAD files). I would also ask if you need to comply with international web server standards though OGC (Open Geospatial Consortium) like WMS (Web Map Server Raster)), WFS (Web Feature Server (Vector), WTS (Web Terrain Server (3D grids). Which means do you want other people to access your data though thier GIS or simply contribute to a industry or national sharing of data. Not many of the GIS server based applicaitons access all of the data from disperate sources live so it depends on your data you want to query and how static it is and if you want to translate it (duplicating the data) before you serve it. For example a Docuemnt managment system could do what you describe but has a completly different data structure to a GIS and would not allow you to do spatial queries. so start with the data and then look for a system that gives you what you want. I would say there are several solutions I can think of out there so you really need to decide on price, interopabiltiy and key functionality.
The ESRI solution although powerful is not cheap and not very interoprable or easy to develop but does offer a lot of power. Other GIS Web servers which are pretty good at reading many datasets live and don't require Middleware and have far more OGC credentials than ESRI are GeoMedia web server (Intergraph), Cadcorp ASC or GeognoSIS (Cadcorp).
GML is a OGC file format based on XML so is not really for serving on the Web although a similar structure is used within OGC WFS which streams vector data live.
SVG is not a geospatial format although it allows you to server vectors to your client browser) I'm not sure if you need either of these.
All the best