Dear Muthu,
Many thanks for your questions.
Regarding The Tamil Nadu Region for growing grapes. In essence you are facing exactly the same issues as Mahrahtra i.e that you are growing vines in a tropical climate and as such the Vines are unwilling to go into dormancy (shed their leaves). This means that you will need to adopt a double pruning methodology with a harsh pruning to discourage vegative growth and encouraage fruitful growth.
Very basically, and we can go into any of these points in more depth you will need
1. An area with soils that have good drainage and not too clay like and soil erosion is not too much of an issue. If drainage is excessive then irrigation will be required.
2. Wind - An area with a good throughput of wind will help to keep various fungal diseases at bay
3. Cool Nights, warm days - You are looking for a large variance in di-urnal temperatures
4. You need to find a place with optimum GDD (Growing Degree Days) and low Humidity (Branas Index for example). These indexes will point the way towards which varieties you grow, or whether your site is not suitable for growth.
If you move towards the points above you will have an area suitable for grapes of the quality required for wine. Regarding your second question about locally grown grapes. These may be suitable - to asses you will need to examine their phenolic content and acid/sugar levels. If the grapes are grown for Table Grapes, be careful because the farmers will be adding heavy amounts of NPK to the soil to encourage fertile growth and increase Yield. These methodologies will not suffice for Wine Grapes - How ever the varieties may be suitable following your assessment of their phenolic content.
In terms of other areas around the region, my guess would be that you would be looking for highlands at least 1000m in altitude, but again this all depends on Mean Temperatures and rainfalls found in potential sites
Puneet