Hi newbie here, would appreciate any expert help and advise on how to buy into S&P500 index, dow jones index and nasdaq index???
First point, you shouldn't do this. It's called speculation, and I don't recommend it.
To answer your question directly, and assuming you are not a U.S. person, these funds track the three indices you mentioned:
U.S. S&P 500: CSPX
U.S. Dow Jones Industrial Average: CIND
U.S. NASDAQ 100: CNDX
These funds are listed and traded on the London Stock Exchange.
What platform is preferable? Local Banks, stock brokers, online Trading?
Generally speaking Interactive Brokers is the best, lowest cost choice. For infrequent purchases Standard Chartered sometimes has merit.
Is there any minimum sum of money to open account?
Interactive Brokers: No.
Standard Chartered: Yes, over on the bank account side.
Any Minimum lot or quantity in order to purchase?
1 share
Yes.
Commission fee and brokerage fee?
Interactive Brokers: extremely low for both the currency conversion and brokerage commission, subject to a reasonable monthly minimum commission ("activity fee") for total account values below US$100,000.
Standard Chartered: minimum US$10.70 per trade plus currency conversion cost (about 0.4%). The minimum commission is lower for the bank's "preferred" customers.
Any restrictions to worry about?
The one that sometimes pops up is that these fund shares cannot be easily transferred in kind between brokers.
Is it convenient to buy and sell? Able to transfer back from USD to SIN$ easily? What are the pros and cons?
They're both fine. Interactive Brokers offers a choice of 5 different user interfaces, and you can choose the one that's best for you.
You should be able to open a margin account at Interactive Brokers if you insist, and they offer extremely low margin interest rates (currently 1.59% interest per year on U.S. dollars). However, see the first sentence I wrote.
Interactive Brokers offers some SIPC insurance coverage, and Standard Chartered does not.
Cash, in any currencies, held at any U.S. broker, including Interactive Brokers, is U.S. estate taxable. For non-U.S. persons the U.S. estate tax exemption is US$60,000, so if you wish to avoid U.S. estate tax then keep your total worldwide holdings of U.S. estate taxable assets (including cash, in any currencies, at U.S. brokers) at or below US$60,000. The funds listed above are not U.S. estate taxable unless held by a U.S. person (who shouldn't be holding those particular funds anyway, even if a speculator).
Another choice is to speculate on the indices you mentioned via the U.S. futures and options markets. Interactive Brokers is fantastic in supporting that other form of gambling.