Write it as a monolith. If you get to a stage where it needs a bigger server than you can afford, run a profiler on it, and fix up any glaring performance problems. Then if it gets wildly successful and does actually need microservices, then you have the resources to pay for the development & sysadmin overhead.

Also by delaying as long as possible, you'd have the maximum information and maximise your chances of getting the split right.