This is something I've given some thought to so I'll offer some reactions and feedback.
Common, Recognized Coordinate Systems
First--it's perfectly fine to start the interment dataset in Excel. My guess is you have some "army" of students, genealogy enthusiasts, and possibly some cemetery employees building the point data with GPS points. To avoid any frustrations down the road, just make sure everyone is taking LAT/LON points. I've seen people take points in UTM before because they didn't know any better and it's like, "great.. now I've got to surmise what UTM zone they were in.." These days most units seem to be in Lat/Lon by default, but the scenario I'm recalling was an older Garmin unit. Once you're ready to publish your data, you'll want to save your Excel data as a CSV file type. ..given all the European names, I wonder how that may impact any diacritics? That's a potential character set encoding issue. Perhaps someone will comment on that point..
Second--I suppose you've got AutoCAD and you know it well which is why you want to use it for the line work. That's fine too, just make sure to setup your project to use a common, recognized coordinate system, this will ensure simple transfer into other GIS systems when you're ready to publish your data. I don't know AutoCAD, so I don't know how to advise you on this point, but assuming you're solid with CAD, I expect you probably know what I'm talking about. UTM 31 North with a WGS 84 spheroid might be a good fit--but you may be aware of a better CRS. Just make sure it's a popular CRS before you're knee-deep in the project, otherwise you'll have growing pains when you're ready to move the data into PostGIS.
BTW, I realize I just said to stay in lat/lon for the points, then recommended UTM 31N for the line work. But that's only for simplicity during the capture/drawing phases. For anyone collecting points it will be more intuitive, and preventing confusion there is important. Then, for CAD, I'm assuming you need a Cartesian grid. Once you're ready to publish the data, you can merge everything into the same system.
Finally--once you're ready to publish--this is where I would deviate from what you proposed in some respects. I agree with translating the CAD data to shapefile to make it more portable. But were it me, I'd use OGR2OGR to load both the Excel data (as CSV) and the AutoCAD data (as SHP) into PostGIS. Next, I'd stand-up a Geoserver instance to serve your drone imagery. With the data ready, I'd use the Leaflet library to handle the web cartography.
Lastly, and admittedly some may disagree with me on this point--I'd write PHP methods to fetch the vector point/line data from PostGIS directly and push them into the map as interactive data layers. Where interactive means the end user can hover/click on the grave plots/points and get super-quick feedback. There are other, less interactive ways to load in that data with Geoserver, itself, but I like to keep the site feeling fast and rich, and it's easy enough to do this with some "bare metal" PHP endpoints.
As mentioned in my comment, above, I'd furture recommend acquiring your drone imagery in the winter so that you can see through the trees (if that's a problem)! Otherwise, while your point data and your line work will still represent the grave site, you won't be able to see grave itself. On some smaller cemeteries with a great number of mature trees, this creates a seemingly incomplete user experience. (Not sure how to describe that, really.)
To put some icing on the cake, it's pretty easy to put together a little search-by-name component that spits out search suggestions as the user types. For an example, check out this demo site I built awhile back for a friend: http://www.solposition.com/ Unfortunately it's a bit dated now, built in Flash using the OpenScales library, but in the search bar, type "wanna" and watch it spit out options corresponding to that street. That's the idea. In fact, in many respects that SolPosition site is exactly what I'm recommending for you, only with your custom interment data, rather than the solar stuff.