I mainly want a site that offers advertising the property, online payment options, maintenance requests & at least a basic accounting aspect.Thank you in advance!

Look at other rentals in the area and see not only how they are advertising or promoting online but also the inside and outside of the rental so you can create a similar environment to try to replicate the same results.

is there a way for me to create an online STR guestbook for free? 

If you know of any online forums or organizations where I can learn from others and expand my network and knowledge on wholesaling or getting my first deal, I’d really appreciate any recommendations Thanks in advance for your help!

When the date came he said he wasn't going to submit it by the online app (stipulated in the lease) but would have it in cash and that we could come pick it up (we live 3 hours away). 

He has no social media, no pictures, no videos, no online presence, nothing.

-I'm fairly careful with communication , but again, I know doing business online is way riskier, but I want to ask: Typically, I would trust someone who seems very in-depth, informative (gives statistics and numbers), and basically would give me thorough steps (including worst-case scenarios and what would best fit my situation)^^would people who have experience say that even with the above, some agents/lenders/PMs would still be sort of out to get your money and do a lousy job?

If you want to put the deposit in a out of state/online bank, then you will have to furnish a bond.  

You can also check their reviews online at Google, Facebook, or Yelp.

To managed it a bit easier (when you grow to more than a dozen), it'll be better to have one online login and manage all 'child' accounts.