I have been able to create a POC environment without 'spending any money', however, Horizon Cloud Service requires the use of F Series cores (for the jump box). These are not available on the free trial, and in order to request these, you do need to convert to the PAYG model. That said though, you can convert - deploy a node, spin up some desktops, do some testing all within the 1 month, and if required then avoid spending anything more than the free $200 trial - providing you then delete all objects and stop within the 1 month.
Im sure you will enjoy the service though, and so stopping shouldn't be something you want to do! ;D
Thank You for confirming, Peter, that most certainly clarifis the concern around the Free Trial amounts not working