Travel in Madagascar is complicated as the road network is rather poor and the country is basically divided in north and south with Antananarivo (Tana) as only connection point. I would very much advise to get the excellent Bradt guide to Madagascar (the lonely planet is significantly worse...). I have done Madagascar completely independently, but you could do without a guide if you hire a car with drive in Tana. Every national park has obligatory guides, so taking your own already is a bit double and your driver will probably act as a guide along the road anyway... Additionally the Mammals of Madagascar guide by Nick Garbutt is really valuable.
Where you should go depends all on your interests, if it is purely sifaka, you could possibly see 6 species in 3-4 weeks, but it involves some travelling... Regardless of where you go, Andasibe Mantadia (Perinet) should be on your itinerary as it has habituated indri and diademed sifaka groups. Another amazing rainforest is Ranomafana in the southern part, which has 3 species of bamboo lemur and Milne-Edwards sifaka.
A place I have not been is the Morondava area, but this has beautiful landscapes (Avenue des baobabs, Tsingy rocks). Here you can visit Kirindy forest (not the national park, but the privately managed one), which in dry season guarantees fossa sightings and Verreaux's sifaka occur here, just as many other lemurs and votsotsa and narrow-striped mongoose. Just north of here is Tsingy de Bemaraha, a national park with beautiful Tsingy rocks and Decken's sifaka and western bamboo lemur.
Additionally you could spend a few days in Ankarafantsika, a national park a day northwest from Tana with habituated Coquerel sifaka and 6 other lemur species easily seen and Madagascar fish eagle. A few hours further on are forests where Crowned sifaka can be seen.
The holy grail: the silky sifaka can be seen in Marojejy but this is more difficult to get to if you are not flying, in the north also Golden-crowned and Perriers sifaka occur, but they are more hard to get to...
Travelling in Madagascar is a slow business, as roads are poor and driving at night must be avoided at all times due to armed robberies being quite frequent... But you can fly to most larger cities from Tana, with Air Madagascar. If you opt for that, book when in the country, that saves you 33% of the costs.