WSSL Seletar is your best option.
Just note there is no approach into WSSL so you have to hold WSSS as an alternate, they are used to this. Generally the weather is fine in Singapore and if there is any CB activity it only lasts for 30 minutes.
Singapore is an easy asian country with the same service and expertise you would expect in USA and Europe.
MAJ Simco
Jet Aviation
Universal
Hawker Pacific
Wings Over Asia
All do handling. Based on a Super Mid Aircraft you would expect to pay $800-1500 USD for handling, tell them you're not paying for airside transport and the stupid communication fee they throw in. $130-150 for Lav/water.
Fuel is expensive compared to Thailand, Malaysia and Indonesia but cheaper then Hong Kong.
Get catering from hotel as the airport catering through SATS catering is terrible and charge $150 to deliver it.
Just get a taxi to your Hotel $17-25 depending on traffic.
MAJ Simco seem to be the best value and Universal probably have the best capabilities.