Another thing I've done is to go onto STR sites, and check out a couple things.

I've heard of mentees who started cleaning up construction areas on a flip at the end of each work day or picking up supplies at Home Depot and delivering them to the job site.

Hello all, just made my account here on the site to get some advice.

Is looking online at sites like LoopNet sufficient? 

I tried to google but could only find vacantion-like sites with exorbitant prices.- Is there a kind of person who could help me with pre-sale improvements (roughly saying, to tell me what is worth replacing, improving, etc and what is not,1 to make the house more attractive for the buyer)?

You can check out sites like Zillow or Redfin to see what houses are selling for and what rental prices look like in your chosen market. 

It's not perfect, but I like to read the reviews from residents on the site 

At the end of the day vet these lead gen sites/companies on a short-term basis, then flip to something else if the leads are not working while having the right expectations.

This is what I did for a cabin I built in Gatlinburg:  We took a few royalty-free photos of the area off of the internet, and submitted them to an art site online whereby they make a painting of the photo.  

Besides land, infrastructure/site work, and the building construction itself, am I missing any other major category of costs?