Scrap value or PARF (80% or OMV - open Mkt value, the origianl base value of the car )is the amount of cash you will get back from the govt if you scrap your car - as an incentive to buy a new one.
COE all last 10yrs. After that time, they can be renewed at the prevailing quota premium (PQP - 3mth COE average) for another 10yrs (or 5yrs for half PQP). IF you opt for 5yrs, the COE can never be renewed again, if you go for 10, the you can go in renewing indefinitely.
When COEs were introduced (80s, or early 90s?), all existing cars were given COEs. Not sure how this process worked, but a lot of old cars had COEs expiring early 2000s (so maybe it was early 90s). Hence you can see 30yr old Beetles with 8yrs COE remaining (renewed in 2000)
To work out the total monthly cost, calc the monthly loan payment, add in the montly depn (straight line from current value to PARF over remaining COE life - well in theory if you keep the car to COE expiry) and then take off the amount of loan principal repayment included in your monthly loan payment
Interest rates are low, and COEs are pretty low as well currently, so best bet would prob be to get something a year or so old, ideally japanese for resale.