My budget is $300 - $400. I live in India.
My doubt is whether, its better buying a new bike or just assemble different good company spare parts (used or new).
I am very new to cycling, so please help...
Don't know about India, but assembling from parts in my end of the world costs a lot more than a new bike. I know of many instances where people buy a "donor" bike for good new parts at a reasonable price. If you can get parts at the right price, it may be worth it, but problems with compatibility between components can lead to a lot of blood, sweat and tears, as well as expense, especially for a beginner. It seems every year has at least one small but significant change that is incompatible with last years model.... Something as simple as seat posts - One post recently someone counted 39 different sizes -size increments are 0.1mm - measuring the hole that accurately is not possible for most of us.
I don't know about India. but where I live you can get great secondhand bikes for good prices, if you do homework and take care. My 14 year old neighbor just got a "new condition" second hand bike for $850 - a new one with equivalent components would have been $2.5K. The previous owner was obviously a fanatic on maintenance and all of the important bits look anything more than a few 100km old. It's not risk free, but its the best way to ride a better bike than you would otherwise be able to.
The other option is end of season and last years bikes - often 1/2 retail price just because the color changed. This is a good way to get into a new bike cheaply.
New Cycle or assemble new parts?? this is absolutely different and both of them have an advantage. New Cycle you just choose one and pay. you can't choose part one by one and buying new cycle is more than cheap.
Assemble new parts You will have a satisfy your self. you can choose anything you like and this is absolutely expensive. because you buying one by one of cycle. so i have some advice if you have really like cycle choose s
First, my own experience is that the cost of the new bike is generally a lot less than if you bought the parts separately.
Second, you say you are very new to cycling. If you did go out and buy parts, how confident are you that (a) you'd be buying the correct parts, (b) you'd have the tools to fit them properly, and (c) you'd have the know-how to fit them properly? I don't mean to be at all derogatory to you when I say the last bit, I'm just pointing out that some of the work will be very challenging to a novice.
Third, I don't know the Indian market, or really what type of bike you're looking for, but your budget doesn't seem that huge. As a third alternative, have you considered second hand? Of course there is an associated risk with this (buying a piece of crap, buying something stolen, etc.) but I'm sure with some research you could reduce these risks. You may even find shops which specifically sell used bikes (and therefore anything you buy will come with some kind of warranty). Shops like this exist in England although I don't think many main dealers bother with second-hand nowadays...too much competition from eBay.