Using SAP HANA via REST only

My aim: to ascertain is it possible or not to have interaction with SAP HANA (e.g. via REST-web services) from outside (third party application).

I am a developer and have a web-application. I would like to amplify functionality - to do some calculations via SAB HANA (as a service), call this calculations from my web-application.

The application is almost ready, and I don’t want to change stack of technology – use SAPUI5, SAP HANA Studio, XS orsomething like. And I don’t want to keep my application on SAP HANA Cloud Platform. Just use calculation as a service via REST (or something like this).

Questions:

1. 1. Is it possible on the whole? Some restriction?
2. 2. Which part of SAP HANA should investigate/use? As I understand, it should be something like OData Services. Right?
3. 3. How can I start? Some advices about first steps
4. 4. Could I try it for free?

Thanks in advance!