buying a 3-Room HDB Flat

Hi everyone, I intend to buy a 3 Room HDB Flat. I have been staying with my parents and siblings for 3 years;and its seriously pack for me and my family to squeezed in. And so I decided to buy a flat for my 2 young children a better place to stay, at least.

I have applied September 2012 Sale of Balance Flats Exercise in Bukit Batok since I want to stay near to my parents home. My application has been balloted with a queue position of 4, which is within the supply of 9 units of flats on offer. The selection exercise is expected to start from Jan 2013.

Fyi, My household income: $1,800;
Purchase Price: $222,000;
I have obtained HLE letter: $79,900 (monthly repayment is covered under mthly OA contribution)
Cash topup (estimated): $ 88,220

Now my question is when do I have to pay the cash exactly, i.e. on which appointment/how many months by now estimately? Can I borrow from bank if my cash is not enough? Can I pay the cash in installments? Are there any other alternatives?

Also, if I were to change my mind, can I reject this offer and renting a HDB instead? and how is that?

Or do you have any better suggestions? I am in a very big dilemma of taking or rejecting the offer.

Thank you in advance.