This is a really hard one.
Your best way to start planning may be to decide how you want to exit.I if u r going to Oz then the overland route thro China may/maynot suit you so,aiming for Kochi(Kerala) or Kolcutta may be the options.Whatever,you will then be able to plan your drive there.
Touring in India is painfully slow so,do not overestimate daily driving,400km here is a VERY full day in most places and like 1200k in Europe.If you have 'loadsadosh'you may wish to see some key places by internal air rather than days of driving.I have done a lot of driving in the North and have been driven a lot in Assam.See Mitchells prevoius message re NE but,this area should definately be on yr hit list.
If that were the case then I would allow 4 weeks to see all of Rajasthan,some of Gujerat and up to Amritsar.
Then to the Himalyas for 2-3 weeks and down to Delhi and Agra,say another week.
It may then take you a week to get to Guwahati(Assam) and do Assam and the NE.Check Mitchell on this but I would say a min of 3weeks just to cover main sites.Do you want to go to Sikkm and Bhutan,if so add time.
From there you could probably get to Kolcutta in 5 days and ship from there.
The other option would be to spend several more weeks touring thro South-Central and shipping from Kochi which is a great place and you will see a lot of India.
Kajuraho should not be missed but unless your a pilgrim the majority of Temple sites do not compare with it or,some of the ones you will have seen already.
If you allowed 3-4 months you will have done a lot but nowhere near all.
There are only 3 things required for driving here:
Balls of Steel
Nerves of Iron
Manic sense of humor
You will have developed all within 3 days of entering;failure of any could be disasterous but hey if you got this far what the hek.