Kohler makes and distributes its own bidet seats in Singapore, so you can be assured of a matching combination as long as Kohler approves a particular combination. For example, they list their Reach Up one piece toilet (K-28633K) as compatible with their own bidet seats. They also offer a simple wash-only “manual” bidet seat that doesn’t require any electricity, the C3-030. There’s also a slightly improved C3-035 model (metal instead of plastic control knob, I think), but if you really want the -035 it looks like you have to import it yourself from Malaysia or elsewhere.
Toto offers some wonderful combinations, too, but they don’t seem to offer quite as much plumbing compatibility as Kohler does. Toto also offers “manual” bidet seats in their “Eco-washer” line if you cannot get or don’t want electricity in the bathroom.
I really like American Standard’s Signature toilet’s rim design for how easy it is to clean. But unfortunately they don’t distribute a one piece Signature in Singapore yet (which would also be easier to clean), and I think they’ve got some bidet seat and/or plumbing compatibility issues. They also seemed to have more COVID-19-related supply chain issues in Singapore, but maybe they’ve resolved those issues by now.
The various Chinese and Southeast Asian brands seem to have some pretty good products now, although I question their own bidet seats a bit. In some cases if you check dimensions carefully you can combine a major brand bidet seat with an off major brand toilet. If you decide to do that then what you might do is bring your preferred bidet seats’ dimensional drawings, and a tape measure, with you on visits to bathroom shops (e.g. in Geylang) to figure out a good match. Bear in mind that bidet seats inherently reduce the toilet bowl’s open “business” area. If you want to “test drive” a bidet seat that’s possible at W.Atelier’s (the Toto distributor’s) showroom’s restrooms.
Yet another potential factor to consider is that it’s occasionally possible to obtain a higher positioned toilet in Singapore. In the U.S. these are called “ADA compliant” toilets (Americans with Disabilities Act), a.k.a. “comfort height” or “right height.” The idea here is that it’s easier for someone transferring from a wheelchair or other mobility aid, such as a walker, to land on the toilet successfully and less painfully if the seat is higher than standard height. Then there might also be a toilet foot stool positioned in front, depending on the situation, to mitigate dangling legs. So if you’re trying to cater to someone with current or future anticipated mobility issues, maybe that’s a good choice. Comfort height averages about 7.5 cm higher than standard height. I believe American Standard offers a couple “right height” models in Singapore, but I’m not sure about others. It looks like their Signature toilet is “right height” and combinable with Kohler’s bidet seats (if not American Standard’s own), for example. But the Signature may or may not have the right plumbing (trap) compatibility for your application.
