Thank you both for the advice. Here is what I have so far for a new loan in 2019:
Create sales receipt for John Doe
Used Car for $2800 mapped to Sales Income
LoanJD -$2000 mapped to Other Asset account
Remaining $800 was applied as "downpayment" and recorded into the Cash on Hand account and deposited to Business checking the next day.
Recurring Invoice setup for John Doe for $50/wk ends after 40 occurences. LoanJD used again as the product for payment and debit of loan account.
Almost exactly what I want, customer just cant see a running balance when they login to pay the invoice as i believe it will just show the invoice total, not my loan balance total. Workaround to that is monthly customized report for that loan account that auto-email customer the PDF.