Ting, if apply BTO, you need to ROM within 3 months of COLLECTING KEYS. Which will be like 3-4 years later!
If you are buying before ROM, you need to show proof that you are getting married when collect keys. E.g. ROM booking slip, hotel/caterer/restaurant booking confirmation.
dont't worry about the ROM thing within the deadline by HDB. I have a neighbour in my new estate, they collected their keys Jan 2009. Till now have not ROM! & he proudly declare on forum that HDB also can't do anything to him.
Anyway, planning your own wedding from scratch is very fulfilling. I did everything with husband, from flowers to carpet to food to designing my own gowns to wine. I loved every moment of the planning with my husband. Actual day turned out very well and we enjoyed ourselves thoroughly. Cheap and memorable day. $$$ doesn't buy memories