If the answer is the latter, is it possible I would lose the money I think I would save by missing deals while trying to learn and upgrade my site myself? 

We are still using all the pieces we bought from those sites to this day.

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)?

I've spent quite a bit of time experimenting with a couple of these sites lately.

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

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

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.

Is looking online at sites like LoopNet sufficient? 

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

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.