I agree with Hemal. I think you should consider a free tier.
Right now, I would not be concerned about support costs. Your platform seems to revolve around messaging, which is relatively low maintenance. Realistically, if you get 10-20K downloads in the first 12 months and a few thousand retained users, you'll have done well. The support load won't be a major concern, and you can figure out the actual cost as you go. If it explodes and you have 100K active users, well, that will be a great problem to have!
Far more important at this early stage is learning so you'll need lots of usage. If you put a trial in place, lot's of people will never try the app and, for those that do, most won't bother continuing. It will be far too easy to simply continue using Whatsapp/Facebook for free. I'd guess you'll get > 100x usage if you keep a free tier, and that means faster learning.
Hemal's other point is critical - if I pay for the app and half my friends don't, that kills my utility. Your pricing structure needs to address this.
Understanding long term retention is also critical to determining if you've built something useful. If you have a trial, there will be a sharp drop in retention when it ends. You'll never know if the 1,000s of people that drop off would have continued to use the app. Think about Whatsapp and Facebook - most people like both, but wouldn't pay for either. Since only a small number will convert to paid, it will now take you a lot longer to develop reliable retention data. It will also take longer to determine if experiments to improve retention are effective. (It will also take you longer to get app store reviews, which in turn impedes downloads/discovery.)
And don't forget that a free user is someone you might be able to later convert. An exited trial user is likely gone forever.
I think the way to go is to have a free tier with a compelling paid tier on top of it. It gives you the best of both worlds - fast learning and revenue. Per Hemal, you can consider Slack's storage limits approach, or adding your own features like supporting money transfers, printing photos, removing ads etc.