Sounds like you would still need a conventional loan on top of that to bridge the difference, but it could provide down payment funds. 

I am not an expert in this area but I'm thinking I could possibly use my HELOC for the land and get a construction loan to build the home and once finished roll it into a conventional second home loan.

If its a conventional conforming loan, it may have been sold and you might be only dealing with the loan servicer not the owner of the loan itself.

There is plenty of other places to do a STR, too many matter of fact that's why in general STR inventory is saturated. 

Most have only access to limited inventory, they don't have the same fiduciary duty and you have to make sure you don't get taken advantage of.

Vedran Sehovic Hi Vedran,  I did some more research and see that VBRO and AirBnB have a lot of inventory / unscheduled dates.  

Tough to answer this question, terms should meet your needs and make sure you can afford the propertyWhy are you wanting owner finance over conventional?

On the conventional side, one of the biggest lenders, I belive they'll give you a few days free, but if you need to extend over 5 days then it is 2.5 bps a day, and if you need a 2nd extension then they double it to 5 bps a day. 1.5 bps is VERY reasonable.

It is a financed offer with a conventional mortgage, and includes a 30-day financing contingency. 

A better option though when you are young is to buy low down house hack but you need a salary to get conventional mortgages.