To make a convincing simulation of a scanned page from a 1700s printed book, you'll need to study and analyze each step of the process it has went through. Since there is a variety in how scanned pages look, you'll have to determine some parameters you can randomize to create diverse samples.
I thought the whole idea with machine learning was to be able to skip (part of) such an analysis and instead let the computer find patterns by itself based on the real-life data presented to it.
To make a simulation of the world and then let a computer analyze your interpretation seems less than ideal to me. No matter how thorough you are, you are bound to make some simplifications. Aren't you risking miseducating your computer? Wouldn't it be much better to use real-life samples?
To me it sounds a bit like building a walking robot and letting a computer study it to analyze how people walk.
(I only know the general principle of machine learning so forgive me if I sound naive.)
If I were to create a copy of that single page you show here, I would simply start by setting up the text in InDesign, manually adjust everything, export an image and deteriorate it in Photoshop. But that's a designer's approach. You need a programmed way of producing varied samples.
Here is a list of subjects I would explore and try to turn into variables. Some of them might turn out to be disregardable and there might be additional subjects I'm not thinking of. This is just a quick brainstorm. Each subject is a rabbit hole of its own.
The raw text used for samples should ideally be from the correct time period or at least follow all the grammatical rules from the time. It should use the correct glyphs (all the different kinds of s's, ligatures etc.).
You might be right that most books were set in Caslon, but you would have to research that. If you find examples with other fonts with distinctively different features, you'll have to include them.
You need to both use the regular and the italic version of the fonts.
It's important that the fonts you use contain all the needed glyphs.
The digital fonts we have today differ a bit from the lead types used at the time which had less sharp details and were designed differently at different font sizes. Bear in mind that many different versions of fonts exist. Some might look more like the original than others.
You need to somehow establish some rules for the layout of the pages which you can randomize. You might need to look at a selection of pages and systematically measure: page sizes, margins, font sizes, leading, positioning of page number, headers, footers, decorations etc.
When we use a digital layout application to set up text, a myriad of typographical rules are automatically applied. Most of these are of course inherited from the typesetting craft from the past, but there might have been some different practices back then. In your example, I notice the extra wide space after punctuation characters. There might be many of these rules to account for. And different rulesets might have been used by different typesetters.
The manually set type have some wonkiness to it. This might be possible to imitate by applying random changes to the tracking and also applying tiny random rotation to each individual letter. But of course make sure letters don't overlap which wouldn't be possible in real life.
When printing letters on paper, the ink will bleed slightly into the paper. What we call dot gain today. Sometimes there will be too much ink applied and the letters become very blobby, perhaps even with small additional blobs of ink close to the letters. Sometimes too little ink is applied resulting in lighter letters which might even have dots of white paper with no ink. This might vary from page to page and even from one side of a page to the other.
This can be simulated in numerous ways. For example by applying Gaussian Blur followed by Threshold. Using a mask with some random gradient to create variation. Here is a quick example:
It might be a good idea to be able to add different realistic paper textures. Also look into adding some wear in the form of folds, dots, water damage and whatever discoloration that could occur.
The paper could have some bumps and dents. It might be a good idea simulate this. For example by applying some subtle, random displacement map.
Furthermore the binding of the book makes it impossible to place the book completely flat in the scanner. Some perspective distortion will occur close to the spine. This could happen in either side of the page. You also need to take this into account.
The example you show looks like a black and white (1-bit) scan (which have been saved as a compressed JPEG). Perhaps most scans are like this, but there could also be grayscale and color images. Some heavily compressed files also have this strange kind of mix between color and b&w:
As mentioned, your example has some quite visible JPEG artifacts:
You might need to take into account that input images could be heavily compressed.