I don't think you can simply run Roon with only a tablet (happy to be told I'm wrong). You can only run Roon with a computer set up as the Roon "core." A tablet only functions as a very fancy remote control. Almost any relatively recent tablet will work as this remote control... see this link https://help.roonlabs.com/portal/en/kb/articles/faq-what-are-the-minimum-requirements.
As for a Roon core, the cheapest option is an easily build-it-yourself NUC. I personally use a NUC with an i3 processor which is fine for running DSP in one location. See this link for would it takes to build NUC Roon core https://help.roonlabs.com/portal/en/kb/articles/roon-optimized-core-kit.