Something that will likely speed your transition is to split your task into as small of portions as possible, and tackle those one at a time from a higher level perspective. For example, if a module is doing some custom authentication, look at what’s available in 2.4 already and see if something now stock can do the job, or at least get you 90% of the way there and serve as an updated template for your code. For example, there are already modules for authentication providers like ldap, content caching, database access, etc.
The time savings on the simplified troubleshooting steps are likely to be far from insignificant, and you would have the benefit of a more standardized/modern codebase. For example, get the server up and running with static content, then compile one module with apxs, get it to load, then work on the related config. Work through any issues you find with this, then move on to the next. This may be a good time to split one large config file up in many smaller ones with includes, etc. Trying to migrate an existing non-trivial site in one step is likely to be overwhelming for most, especially for a novice.