Before proceeding further, you should be clear about the following

1. What is your objective- Is it commercial, hobby / time pass or is it experimenting?.

2. Why have you not considered buffaloes- any special reasons?

3. If your objective is commercial, then- you may even go for high yielding crossbreds. Even Buffaloes also could be an option.

4. Considering the changing climate and suitability of indigenous breeds in thriving well in Indian subtropics, it could be advisable to rear indigenous cows. However, if you have to look in to the commercial part of dairying, getting high producing indigenous animal in affordable price is a tough task presently. Moreover, there is a possibility that the performance of many dairy breeds of indigenous cattle may get disturbed in the high humid conditions of Kerala (many parts). So before proceeding further you need to experiment in your own situations.

Considering the  above, you may start with few HF and Jersey crossbred cows (please avoid pure Jersey and HF) and few indigenous animals (Sahiwal or Gir would be good choice to start with as getting good Red Sindhi is difficult). High yielding Kankrej animals are also available and the price could be less. However, these animals are very hardy and at times a bit vicious. They get adjusted soon in an open system.

We are not sure whether you have any prior experience of dairying or not and therefore suggest to start with not more than 20 animals (may be 10 crossbred and 10 indigenous) in beginning which can be further increased based on the experience. While procuring get the help of a qualified veterinarian and get the animals tested for some crucial diseases like- Brucellosis, TB, Para TB, Mastitis etc.

You need to look in to the source of fodder as it will be difficult to maintain 20 animals and their followers by producing fodder in 1 acre land. May be you have to procure / arrange for fodder from other sources. Cost of fodder (both green and dry) and feed would be crucial determinants of the net profit.

If you are not already trained in dairying or do not possess previous experience of dairy farming, it is always advisable to undergo a short course on commercial dairy farming from your state veterinary colleges or may be in ICAR-NDRI, Karnal/ Bangalore. This will help you to understand the dairy business from all aspects.

You may also contact officers of Kerala Livestock Development Board/ State Animal Husbandry department/ Veterinary University, for further suggestions and help.