Short definitive answer - No idea.
Longer speculative answer:
(1) Singapore operates a mature and multi-layered Integrated Air Defence System (IADS) and has an air force that is fairly capable. The current IADS system lacks ABM capacity but we do have a Russian designed short range Igla missile in our
SHORAD arsenal. The Igla missiles are mounted on M113A2s and the Singapore developed Mechanised Igla comes in two variants (click
here for fact sheet). So I can't say that we will not use Russian SAMs but it is unlikely that we will consider the S300 due to integration issues with our IADS.
(2) Given Singapore's small size, we may have to consider a sea based platform if we want to guard against ballistic missile threats from North East Asia (i.e. North Korea). Given that we are within artillery range of Malaysia and Indonesia (and that these two countries do not have ballistic missiles, yet), any current potential local threat is more from rocket artillery and not ballistic missiles. This will change and I believe, in time, we also need to acquire some capability to defend against cruise missiles and low flying UAVs too (as China proliferates her technology in this area).
(3) The current Aegis/Standard Missile combination is too expensive for the RSN to acquire (however, I'm watching developments of the sea based PAC-3 closely) and we also have six Formidable Class ships who use the Aster missile. The Aster 30 Block 1 (mentioned by swerve in his post), when it is developed could be a candidate and offers the advantage of some logistics commonality. Our over-riding concern for ABM capability is cost and we have a limited amount to spend for missile defence at the moment (as we have other priorities) and there are also other candidates like the Israeli developed Elta EL/M-2080 "Green Pine" early-warning AESA radar/Arrow combination or even the EL/M-2248 MF-STAR/Barak 8 combination (given our G550 CAEW acquisition). Singapore is very interested in longer range radar technology and the radar is a key component in an ABM system.