Use seller financing to hold the note to increase your profits, not sell the note to lower your profits.IRR would be the best way to calculate your ROI.

Given this is your first deal you may have to put some money into the mix and your lender may go with a lower LTV. 

Basically you are paying more upfront in exchange for a lower long term interest rate.

And the broker can discredit other, more honest brokers by saying those agents who suggest a lower list price just want an easy sale.However, if a Broker gives you a list price that's much higher than other brokers, usually it's a trick.

-Check and see if there is an easement on an adjacent property for a future driveway access to your landlocked property-If there is not, ask your neighbors nicely if you can add a driveway access to your property on theirs-Try to sell the land to an adjacent owner

Wholesaling seems easier in lower priced markets from what I have seen .On the plus side if you actually have a deal it’ll be easier to assign it since there is a lot of investor activity now and it seems everyone wants to be in real estate .

However, if the attic is accessible on its own and is set up for heat, then it should be able to be counted as a 3rd bedroom. 

and if so is there a lower limit on the amount of cash flow I should expect on any given property?

I took one on my primary residence, paid no origination fees, and make no payments on it unless I use it, and only pay $70/yr maintenance fee, so if I never touch it over the life of the HELOC (10 years) I'm only out a total $700 in 10 years, which I think is reasonable for the access to the cash.

Find a partner who can put in the capital or There are private/hard money lender that will let you put 10% down payment and  they will finance 100% rehab cost. keep in mind the rate will be a lot higher and they charge point (2%) but they can close fast and you might be able to negotiate the purchase price even lower.