Hi you can only apply for a new BTO after the 5 years MOP of your Resale Flat. Once you have completed the 5 years MOP, you can then proceed to apply for BTO Balloting. If you manage to successfully ballot for a new BTO, upon receiving keys to it, you will have 6 months to sell away your Resale Flat. All of the above is assuming that the Resale Flat is your first subsidized housing.
I have not done this myself before but personally know of someone who did. I think there are 2 challenges you might face. Firstly is the burden of obtaining and servicing mortgage loan by the Owner and secondly is the level of trust between you and your fiancee. If the Owner's income is sufficient to obtain the required mortgage loan then this will not be an issue.
It depends on your risk appetite and comfort zone as an investor. Investing in Overseas properties is very different from investing in Singapore properties. A few key points that you will have to take note about Overseas properties: 1) Political and Economic stability of that country 2) Currency fluctuations 3) Taxes and other miscellaneous fees 4) Ownership eligibility 5) Remote management 6) Legal system 7) Securing loan These are just some of the points. Some of the popular countries that Singaporeans like to invest in are Melbourne, London and Bangkok. UK has fewer restrictions on foreign buyers whereas in Australia, foreign buyers can only purchase new property and not resale. And in Thailand foreign buyers cannot wholly-own land. As you can see buying a Overseas property is not as straightforward as buying a property in Singapore. However, it does not mean that one is better than the other. Do your research on the developer and market.