I'm assuming theres a good chance JT will be leaving at the end of next season when his contract expires and he'll be close to 34. That will leave us with two proper centre backs, and Iva who is usually deployed as an RB who can obviously also be used at CB, but that would then leave us with only one right back (assumeing Ferreira will also be gone by the end of next season).
So yes, I think we should have more than 2 top centre backs. Look at United and City, they have at least 4 CB's each that they can rely on to start, and Luiz and Cahill have both already picked up their share of injuries since moving here.
Not to mention the fact that many people still don't trust Luiz at CB, and Rafa has already showed that hes contemplating giving him gametime further up the field.
I rate Fellaini and no doubt he'd be a great signing for us, I feel like we have been missing that element of power and drive in our squad this season, and really have been lacking it in the midfield since Ballack left and Essien became a shadow of his former self. That being said if he was to fit into the team, surely it would be at the expense of Mikel, as I don't see him pushing our trio out of the team, nor ramires. But the problem with that is that both Ramires and Fellaini will want to be getting up the field on and off the ball, we need a proper holding player if we're playing the 4-2-3-1, particularly if Ash and Azpi are getting up the field, or we'll be horribly exposed. That is why I think he's not the player we need, but hey since when have we bought players we actually need.
And I agree the striker doesn't have to be the best in the world, I said Falcao because the OP recommended him, and he's been recommended with us a lot in the last year or so. Theres plenty of options we could probably get for half the price, the likes of Ba, Higuain, Llorente and Pato come to mind, but considering money isn't really an obstacle for our club I don't see why RA would be particularly bothered about saving 15-20 million if it means purchasing a striker head and shoulders above anyone else in the world right now.