Actually for the prices and downpayment amounts, you'd have to
go down to the dealer yourself to ask. The prices change often, so
you'd have to go down and find out.
Your monthly payments depend on how much you borrow from the bank,
how long you want to repay (number of years) the loan, the interest
rates given by the dealers etc etc.
As for off-peak cars (they aren't called weekend cars anymore)
there is an upfront $17,000 dioscount/rebate on your COE. So if you
get your COE at $27,000, you only pay $10,000 for it (note the
paper value is still $27k)
The $20 coupons can be purchased from LTA/URA.
Insurance premiums are the same as normal cars. As for how much,
heh, it's up to the actuaries to calculate how much risk yopu are
to them 
But if you are
married I think that would reduce it by a lot!
You can calculate your road tax online at onemotoring.com.sg...
there's a formula there, and for off-peak cars, there's a $800
discount on your road tax per yr, so expect your rd tax to be less
than $10 per month.
Seldom would you find used OPC... most of the time the 2nd hand car
dealers will convert them to normal cars and sell them.