Well, i assume the 7.000 is provided you buy a new MK IV at full price. SO yes the 7.000 seems alot, but maybe it isnt. The real question is really, how much do you need to pay additionally to upgrade?. Remember to have DVB-HD fittet in your MK III also costs alot, so as other mention, it really depends what you need. If you do not use DVB-HD or bluray and only want to have the new panel with 100 Hz, why not just get the MK IV screen without bluray and DVB-HD, this will save you some money. You can as mentioned, then buy a PS3 for Bluray playback etc.
The issues experienced with the new BV-7 is really only issues related to the DVB-HD and Bluray.
I would really ask your dealer how much he wants for a non Bluray, maybe non DVB-HD BV7-40 MK IV if you swap.
Regarding your speaker choice, i would definitely go for the 7-4 regardless if you upgrade or not. The 7-4 is a mono speaker designed to be center in a surround setup and will therefore perform so much better in your setup.
Finally, i would never swap my 7-40 for a 10-40 as the BV 7 is far superior in terms of connections, processing etc. Take a look at this thread.
http://forum.beoworld.org/forums/t/32048.aspx
/Ehlerz
BV7-40 MK IV, BL5´s, BL3´s, BL 3500, BL7-4, BS3000, BC 6-23, BV 1, BS3.