I think long range systems to be considered should be cruise missiles and MLRS
It should be remembered that MLRS systems are mainly used to attack infantry and lightly armoured or soft skin vehicles, not heavily armoured vehicles. Also,, if Singapore were to purchase the MLRS, its range should be longer than 40km, since the current artillery in the SAF inventory can already achieve fire to 40km, and at a much lower cost. I must say that the Brazilian ASTROS II pruchased by the MAF looks best after a quick look at various MLRSs on the
www.fas.org/man website. The main advantage a MLRS will have as compared to an artillery gun with a similar range is the MLRS ability to deliver massive amount of firepower to one place in a very short period of time. One MLRS could deliver the same firepower that only many artillery guns can in a short period of time. However, for sustained operations, a gun would probably fire better and is alot cheaper since a 155mm shell, even Extended range ones, will cost much less than each rocket the MLRS fires.
Cruise missiles can be readily developed from anti-ship missiles in many instances, including the Harpoon. In fact, the US already has a cruise missile SLAM and SLAM-ER developed from it. I would say that a cruise missile is much more likely to be used as compared to a SRBM, simply because a cruise missile is able to attack individual targets while the SRBM would simply blow the hell out of any where it lands in in a 5 to 10 km at least, depending also on geographical factors of the place. A cruise missile is much more practical, although is failings are generally a speed of less than 1 mach and since the target is generally far away, this allows the target to manoevure away or a good chance at shooting down the cruise missile, especially if it does not fly very low. Consequently, cruise missiles are used only against mainly stationery targets.
IMHO, if MLRS is needed, the launch vehicle should be an indegenious production or modified to fit Singapore's needs, but rockets can be either purchase from elsewhere or developed from existing ones, especially if a range of 90 km like the SS-80's, or even more, is wanted. It should however also be noted that there is some point in the range where the rocket would become uneconomical in terms of the size of its warhead as compared to the price of its rocket engine
Cruise missile option can probably be the SLAM-ER, since it has commonality of parts with the Harpoon and makes handling them cheaper and easier.
my two cents