"I'll have" and "I have" are both OK. Ordinarily, present continuous is OK, but we usually don't use continuous tenses with stative verbs like "have". One weird exception is when "have" is something you're looking forward to:
"I'm having chicken for dinner tonight"
"I'm having Bob over as my guest tomorrow."
However, people usually aren't excited about meetings, and it sounds weird to use "I'm having" next to "meeting."