Pretty sure mortgage debt is the cheapest thing out there to borrow from. Then probably line of credit. Then business/car loans..... worst are credit card of course.
As for LoC monitoring, I don't think they care what it's used for as long as you can it off as prescribed by the terms of the LoC. LoC payback terms are not the same as credit cards. Credit cards you may only need to pay off $20 for a $2,000 monthly bill. As long as you pay off that $20, they are happy.
With LoC, the terms can be totally different. I have over $50k of LoC too and only used it once a long time ago. The payback terms was that I pay back a minimum of 3% of the balance per month. I never bothered trying to revise it or anything since my LoC taken out was small. But if you take out $50k and have a similar pay back schedule, you may need to pay back $1,000+ per month.