I'm just a small-time landlord (20 units), and I like using Cozy.co for my application and screening, but I want to ask more questions on the applications.I've looked into some other cloud software programs but haven't found anything that would let me customize the rental application on their platform.

I know speaking with contractors is the most efficient way to get accurate costs but it is too preliminary to do so.Any websites, softwares, books, etc to find some general costs that I can base off of?

There is rental software company and lot's of others.

Looking to get into a property management software and wanted to see if anyone has any suggestions on which one to use. we currently manage 24 units and have been using excel and quickbooks.

Monte Carlo sounds almost too much for me haha you need a software to run that kind of simulation.

A great alternative can be to use self-management software.

Excel or some other software?

With that being said, most of the features you mention will come with much more expensive software that you pay for.

There are software systems out there, like Argus, which can handle a lot of this, but I have a feeling you don't want to pay for this capability. 

I use Appfolio software.