Answering your queries seriatim:
1. I have been offering services on a particular name say "ABC" for the past 5 months without having a trademark. If someone else applies for a trademark on "ABC" and sues me for using his/her TM is there a possibility for my firm to defend itself as we were the first to use the name(based on domain name registration, etc).
Yes…. You can defend yourself based on the domain name registration. To be on a safer side, please conduct expedite search in the TM Registry for your trade mark. Your TM attorney will advise you further on looking into the search report.
2. My doubt regarding website is not yet clarified. If I get a TM for "ABC" is it obvious that I am the only one who can have a domain by name www.ABC.com? Or putting it in a different way, if I get a TM on "ABC", if some XYZ starts a website by name www.ABC.com, can I sue him?
The moment you get the “TM application serial number”, you may show the symbol (TM) to your mark on your website. This will show that the TM is linked to your website too.
I’d strongly suggest that you also register “ABC.com” with the TM Registry. This will solve the problem.
3. If some other firm is dealing in similar products/services and launches its products/services with a very identical name though not exactly the same, can I take action against it?
What is to be seen is “who is the first user of the mark or the deceptive mark for the same products / services”? If some one starts using a deceptive mark identical to your mark, you are open to take action in the court of law… even if your TM is not registered. Your TM attorney will be able to guide you in this behalf.
BTW, a partnership firm can register its trade mark. When an individual (who is not a legal entity) could register, there is no question or bar on the Firm registering its TM.
Trust this clarifies.