Girish -- Unless you are fairly certain that you are only going to be be doing commuting on your bike, and pretty much no other riding (longer rides on the outskirts of the city, or out of town), you might want to consider other factors at this point -- since you are looking at upping your budget to ~Rs30K. For the commute as well -- your commute might change in the future, so you should_not (IMO), focus too much on a specific route.
I would categorise a commute as a not too long ride (subjective, but say not more than ~25kms one way, which is quite long for a commute), through city roads / traffic. The road condition can be considered fair, and you can say there won't be much of climbs (hopefully you can avoid all flyovers). (I am familiar with the incline, going from Marathahalli side, towards Manipal Hospital; just checked on
cycleroute.org -- looks like that stretch is between 5-7%.) With those factors, it is not so much speed that is a issue, but overall ride experience that one would look at.
A good single speed, like Purefix, should give a nice, zippy ride, w/o the overhead of gear maintenance. And while I have not really ridden one for any considerable period of time, I think it will give you a much 'purer / free-er' riding experience. While changing gears does become second nature on a geared bike, I think you will realise with a single speed that changing gears is after all, a chore. Atleast till you hit that next climb. ;)
You may find some stretches tough to handle w/o gears, but in your regular commute, you can just expect to get used to them (as in, your fitness will improve). Also, since the distance won't be much, a climb or two usually will not matter so much. (In some places like Banshankari, there are some roads with fairly steep climbs... those, or as a said, flyovers, might be an issue.)
That said, if you go for a good hybrid, you don't have to worry much about the gradients etc. on your commute, And you can also look at doing long(er) rides, out side the city -- incase that is something you think you might want to do at some point.
If budget is a fairly important factor, do check if there are any good used bikes up for sale. If you get a good used Trek or Bianchi hybrid (just to name couple of good brands), within your budget, it will be a good option.
The Montra bikes do seem to be good; not sure I have seen the Celtic model. I did recommend a Montra MTB to a friend, ~1.5yrs back. (He has not ridden in much, so I don't have specific feedback on how well it holds up over time. but I don't think I will hesitate to recommend that one, to others newbies, even now.) Going by what I have seen in the Montra bikes -- If they have 2-3 similar bikes, see if one of them has particularly good components, for not much a higher price compared to the other similar models. That may make a particularly good buy.
Do try to get a test ride done, for whatever you are picking up. With a used bike, you might be able to get a longer test ride. :)
Best of luck with the buy,
-{db}.