Smart casual clothing for autistic people

Hi everyone,

Not posted here for a while but I have a new job, probably the most professional environment I will ever work in, and I've been told the dress code is "smart casual". 

I really struggle with clothing unless it is jeans and t-shirt or comfier. What can I wear that looks smart but is also extremely comfortable? What do other people wear in professional office environments? I'm also carrying extra weight and now around a size 16, so I also don't want to look/feel huge. 

Any inspiration is appreciated. I have until Monday to buy something.