Hi Sachin,
you can find the neck plates etc on ebay, or via Australian Luthiers Supplies or real parts etc. they tend to be a fairly standard size.
https://luthiersupplies.com.au/guita...ar-neck-plates
They also do the Screws:
https://luthiersupplies.com.au/neck-attachment-screws
you may find cheaper options on eBay, but in terms of quality, they might prove more a liability.
The typical screw size is 4mm (3/16" in the imperial/us standard). I'd use a 4.5 mm or 5mm bit to do the body holes, this allows for the screws to go through but not bind up in the body holes.
You want the screws to bite into the neck heel only.
IN terms of the hole into the neck itself, the recommended guide hole is 3.3mm, but this is based on tapping threads into metal. A 3mm drill should do if you run it in and out once or twice.
When you go to install the screws or test fit them, make sure you lubricate the threads with wax, or with soap, just rub the threads with a piece of candle wax or a bar of soap before you try screwing them in.
I would wait until you have a suitable sized neck plate, then plan your holes on the body according to that screw pattern.
Once you've got the body holes drilled you can align the neck and clamp it in place then use the body holes to mark the screw hole location for the neck.
Hope this helps!