For a first BTO housing, how much do you recommend to budget for renovation? Would getting OCS be cheaper be a wise choice?

New flats tend to be a bit cheaper to renovate (although it depends on how extensive you want the renovations to be).

Do note that many of the quotes you receive will start from about $30,000 - this is because the maximum reno loan amount from banks is $30,000 or six months of your income, whichever is lower. But this doesn't mean it's impossible to do it for less - I've seen 4-room flats renovated for just under $15,000 that still look great.

Speaking strictly rom personal experience, most of the people I know end up spending $33 to $36 psf renovating. It's my general yardstick.