Sissoko was one of the best n°7 in Fm11 but in FM12, he's became an average player unfortunately.
M'Vila is very expensive (around 25M£ but he doesn't ask a big wage), unless Rennes have bad results and take him to the transfert list (then, you could have him for 12M£ or less) so wait for it during 1 season, 1 season and half.
If you find it too expensive, Fellaini from Everton is a very similar player (in lots of my savegame, he's in Golden Ball List each year!). Biggest wage but cheaper to buy