Should I send my 18 months daughter to a half-day or a full-day pre-school/enrichment centre or should I wait till she's older before putting her in for such long hours?

I guess, you should wait till she is a bit older for the full-day pre-school. You can kick off with a half-day. She would take time to adjust there and you yourself would not want to put her in full-day. I remember putting my daughter when she was almost the same age as your kid and she cried so much for initial few days that I had almost thought the idea of picking up a job. And even I had just joined I had to take a 10 day break to get her settled.