Concerned that if I use it as a downpayment, I'll have twice the amount of debt on one property and it's all riding on my tenants ability to pay rent during these trying times.

Doing things this way keeps your ability to execute in level with your investor volume. 

If you have the ability I would go for 20% down payment in our area. 

There are a lot of great options out there; aside from providing the basics (ie. listing tool to get the property published to 10+ major sites, scheduling tool to schedule viewings, tenant screening with comprehensive applications which include nationwide criminal background checks + credit checks + eviction history, digital lease creation tool, rent collection tool, and maintenance tracking tool), some services also offer your tenants additional capabilities which makes it an easier sell on your end when getting them to transition over (ie. their ability to report on-time rent payments to the credit bureaus; this can create an added incentive for making sure they pay that rent on time).

But, those rules of thumb and shortcuts will be specific to your location, the types of houses you're doing, the finishes you use, the types of contractors you're using, your negotiating skill etc.In other words, the shortcuts and rules of thumb that I have for my houses in my area with my contractors and my finishes and with my negotiation ability are going to be different than yours.Learn the process, practice, and it will get easier.  

The next thing I'm planning is to have an ability where multiple contractors can bid for hot projects in real time and i.e., upload photos, send/rec bills, etc. 

If you have the ability that is what I would do if your buyer needs to be in by the end of Feb.

Lots of old houses were built before codes were enforced so we see lots of things like missing headers or strange methods for hanging joists or cut out structural elements for heating ducts and things. 

Primarily because if you use the money and refi out later the loan follows the refi property and pays off the heloc which gives you the ability to use heloc funds again.

I couldn't get financing for deals at first for very obvious reasons, but I have this ability to solve big problems in creative ways.