You can take Apollo Munich Health Insurance - Maxima Product. It will cover three of your four requirement - Mediclaim, Hospital Cash and Critical Illness

Why go for Apollo Munich?
1- 100% Lifelong Renewal.
2- 4500 Hospitals in over 800 cities.
3- Pre-existing illnesses under OPD Benefits, covered: If you need medicines or a Doctor consultation for any pre-existing illness, it’ll be covered without any waiting period.
More Info - http://www.apollomunichinsurance.com...out-maxima.pdf
Mediclaim policy should cover higher category hospital rooms which are usually between 7,000 to 10,000 per day in good hospitals in Mumbai.
I have seen many mediclaim policy's that restrict the room charge to either 1% or the lowest single room.
So maybe a higher mediclaim policy of 7.5 to 10L is required.

Some policy says pre existing covered after 4 years some say 2 years, i am sure there will be conditions attached to this. can someone through light on this please.
In some companies there are multiple TPA, you can choose from. But It's the insurance company which pays the claim so the policy and coverage is the most that matters.
Do you think i should take a separate one or does any mediclaim have it in built.
I think build in would be cheapest and also you need to contact one TPA/ insurer in case of claim.
Critical illness
which is the best, covering maximum common diseases, some cover a lot but mostly you don’t get those diseases during a lifetime, so no point buy a policy which covers 100s but not the common one.
As diabetes is already pre existing, does this critical illness policy help,
Almost all the health policies cover similar Critical illness disease. Diabetes is pre-existing, anyway diabetes is not part of Critical Illness disease.
are medicals necessary
Yes
Do you have to declare diabetes.
Yes. you need to declare