I would like to hear some suggestion. I have no idea how to do a project like the one below. Basically I will estimate some equations separetely, and I want to build a system of all estimated equation to produce a economic scenario based on the equations estimated, inputs for the model, and some definitions.
For example, imagine I want to forecast the following model:

Phillips curve that depends on past inflation, expected inflation, output gap and a costpush variable, like commodity prices.

Output gap equation that depends on past output gaps, real interest rates and global growth.

Expected inflation that depends on output gap, past inflation, and a inflation target.

a definition: real interest rate = nominal rate  the expected inflation for the next 12 months

Inputs: Global growth, nominal rate, commodity prices.
I would estimate each equation separately, using a OLS or a 2SLS.
How can I bring up a model like this for estimated different scenarios based on different inputs? Are there any example?
I used to work on EViews, where you can append and merge components to a "model", and then compute the forecasts. How can I do it on R?