I have a beetle that I have just finshed upgrading the interior in, It is a 1970 1500 beetle. I have put in two times MOMO T-2000 bucket seats
http://www.momo.it/. The reason I choosed to use MOMO was that they have anchoring points in the bottom of the seats that you can bolt straight into. I was able to use the stock seat rails and just weld a bars across them to put the seat bolts through.
With regards to the stereo, you have a huge advantage when it comes to car audio simply because it is going into a beetle. The beetle provides a very solid environment for car audio because of its soild steel construction. You will be supprised how much base you can achieve buy using high quailty audio components and a good amp. I am running Infinty 6" by 9" three way speakers (300W max) in my rear parcel tray and 6 1/2" inch (300W max) Infinity component speakers in the quarter panels behind the drivers seat. The tweeters are mounted under dash, near the top of the stock carpet... all the wires are hidden behind the carpet. The whole system is run by a 85W per channel by 4 channel Alpine amp... I know it is not the most powerful amp on the market by at the moment I don't even have it on half gain and its very loud. I have thought about putting in an sub but when people get in and hear the stereo in action they ask where the sub is hidden... when I tell them that there is none they don't believe me. As I said above the beetle is a great acoustic shell for audio... proably the worst thing you could do is over do it.
If you are getting a new head unit, get something that will play MP3 as you can put up to 13 CD's on to one disk... very handy + a stacker for good measure.