The one major drawback is that is commuters will have to spend 20 minutes (minimum) just driving to a highway.Is it a good time to buy? 

You just cannot do this very cheaply and easily in a city, with many more regulations and fees, than you would have in a small town or county land.The most obvious drawback is isolation.

The obvious drawback to this approach is that you would pay two sets of closing costs. 

This is a large drawback of the structure. 

Are their any draw back or restrictions?

The major drawback though is it takes about 5 business days to receive the rent but on my loans I have a 15 day grace period so it always gets in before any late fees. 

The drawback of the simple ROI calculation is that it doesn't account for the time value of money. a 10% ROI in one year is not the same as a 10% ROI in 10 years.

There are significant drawbacks to this though so make sure you talk to an attorney and an accountant.

That is such a good idea for most, especially as the "benefits" of a HELOC come alongside a number of significant drawbacks.

I agree with many things that you expressed:Being the project manager (GC) is a challenge, however if you have someone other than you in that spot, you MUST keep a watchful eye or you can get burned (ask me how I know).Flipping low end props has draw backs (neighborhood, cost/value restrictions on the level of rehab and still keep it profitable, rehab lenders aren't interested in small loans, and maybe even the unappreciative or qualified buyers).