Pages or posts for reference site

There are 2 things here:

1.SEO:

There is absolutely no issue in terms of ranking when it comes WP pages vs post. As long as you got rich content either should be fine.

2.Website Structure:

Pages vs Post could come under your website planning and future consideration. For general understanding, pages are generally for website page and posts are for the blog.

Note: Also, it's good to have fewer pages with rich and good length content instead of having 100s of small pages.

Before I get too far down the rabbit hole should I make this kind of content as pages?

Doesn't really matter as long as you remain consistent with it. Personally, I would make this content into a custom post type and that would allow for custom taxonomies to better handle navigation and SEO issues, but this isn't necessary.

The fear is that eventually I could have hundreds of these pages, is that a problem?

Nah. Again, as long as you are consistent about which content container you use, there's no benefit nor penalty to using posts over pages.