If you choose to leave a small balance i would suggest no more than 30% of your total limit.

No More.

That’s over 121k every year, for the rest of my life... and I'm just getting started.By the end of next year I'll have this same portfolio double in size with no more than the original 40k.

No more airport>hospital>hotel>repeat.I can spend time with family, friends and church. 

My theory is that closing costs for a loan should not be any more than for property closings (ie. no more than 6%).

Yes, the fact that they recently had to adopt new rules for investors (investors can get no more than 5% of any single property per account, frex) and that even with these rules in place you have to act fast to get in on new properties tells me that they have a pretty popular model.

If I max my Roth 401(k) contribution -- let's say I can put no more than $30k into it based on my income -- can I put an additional $5500 into a traditional IRA and roll it into the 401(k)?

In this case, if the market rate for rent of that home is $650/month, then you should pay no more than $65,000 for the home.  

Orange peel is a breeze to match up, no more ugly drywall patches. 

I'd use no more than $255K for a working estimate.Something that will be slightly confusing is you will have to pay property taxes up to the date of the closing.