My personal residence payments are less than 15% from combined personal income.

Additionally, finding a mentor who's achieved success in buy-and-hold investing can be incredibly valuable.

Don't forget income is actually can rise faster than the rent.

The up side on large down or all cash is you minimize your risk plus fewer properties means less work and less headaches. 3 properties with large debt or 1 property low debt or paid off generating the same or more income as the other 3.

See how that feels... the net is the same (provided you can deduct the interest against the interest income earned, I get it again, but...).

Depreciation for tax purposesThe amount of annual depreciation OVERLY offset my net income on the property.

Additionally, what are the typical maintenance costs for these machines?

So many landlords set up these types of additional escrow accounts and fund them monthly to try and anticipate those expenses - but these are separate from the lender's escrow for taxes & insurance.All the best!

Have they ever had to call additional capital from investors beyond their initial investment? 

Utilities, taxes (property, local vacation, & income), landscaping, furnishing / decorations / amenities, pest control, & supplies (soap, TP, etc) are the big ones on my STR calculator that you didn't mention.I'd also recommend using the enemy method to determine expected income.