Have 35k saved so far.  

A more stable of building wealth is to have as many rental properties as possible before I'm forced to retire from my physically demanding job, The safer route seems to be to pay off the mortgage rental as fast as possible, which I'm doing.

Then, I'm not sure if I want to go the safer route and save for another rental (from the income of my full-time job and the 1st rental) or finance the 2nd rental (and have two rentals and my income pay it off).

there is just a ton of debt you have to take on and as we age we get debt adverse.. now in new construction we take on debt of course but we retire it.and yes its a job.. but when its in the right market cycles boy is it a good one  Can make more on one new build in 9 months than a rental house in 20 years.. so its just scale. and what you want to do .. 

Changing to a 15 year loan would mean your grandma has ~$100 more each month to put toward bills, which can be helpful if she's retired and on a fixed income.

Estimating we would need $20K to $30K for a $200K - $300K property.ORShould we keep this SFR home paid off to maximize its cash flow and use other sources of financing (personal savings or piggy back a home equity loan on a conventional 30 year mortgage loan) to finance the down payment on the new investment? 

Is your goal to add a stream of income or replace a full time 9-5 gig, to save a certain amount of money, retire early, etc.  

Writing of 2 home offices (or 4 possibly, in your case) would be a huge red flag for the IRS for the benefit of a few dollars in tax savings.

I do not have a lot of money saved up but want to buy a property ASAP.

I can continue saving cash and wait for a deal or a reduction in pricing.