I would suggest going for a Pioneer or JVC single din HU which has USB support and AUX support. Installing a 6.5" speakers in stock location in front door pads of M800 is a bit difficult as it requires some alterations to door pad. My guess is you can easily fix 4" coaxial speakers in stock locations. The problem is finding good 4" speakers as few manufacturers provide good 4" speakers. With all respects 5.25" and 6.5" speakers are much powerful and sound much clearer and louder but fitting them in M800 would be a big headache. I don't think you can fix 6.5" speakers under seat in a box.
Need to check if 5.25" speakers can be fitted into stock location in front door pads. Do check with your local car audio installer. If yes the go for these:-
Polk Audio DXi5250 5 1/4-inch mobile/marine audio component system DXi5250 - Car | Polk Audio®
If not better go for 4" speakers in stock location.
Polk Audio DXi400 4-inch coaxial mobile audio loudspeaker DXi400 - Car | Polk Audio®
For rear 6 X 9 coaxial speakers you should mount them in a ported box to get as much bass as possible. Don't you tray as it will cause rattling in future.
Infinity - Primus 9603i - 6 x 9 inch Coaxial Speakers Infinity - Primus 9603i - 6 x 9 inch Coaxial Speakers (300 W) [Pair of Speakers] - Buy Online @ Snapdeal.com