@Roger Devore, without knowing the condition, I would say a lot of your assumptions could be aggressive.  

I’d say a long term rental is the most “passive”.

The comparision is with, say, a high tax city like Philadelphia, PA.

Here is the question of the day, do I spend my $70k from my solo 401K and purchase a house at auction or do I leverage that money and purchase say a 4 unit with a mortgage? 

I'd say a big part of an agent's job is to make sure that the investor is making a wise long term decision and that they understand the risk that they're taking on.

The comps I looked at say a ARV of around 507k isn't unreasonable.

Putting 20% down on say a 60.000.00 property would get you a decent house with plenty of money in reserves.

If I rented it out today, I estimate I'd cashflow even or slightly positive to start, but of course I'm worried that I won't get as much per month in say, a year and a half.

So if he shows up on site, needs a $100 Item say a door, then I am now billed $110 door plus the whole time spend on the door from showing up until leaving after the install so including the shopping time.

You'll just need to be prepared to do some work after the purchase.If you look for houses with more bedrooms, say a 4 or 5 bedroom house, you can rent out the additional rooms and make some money that way.