Troy,
I'm in the military as well, and have a rental (bought as primary about 12 years ago) in Norfolk. Since we bought as our primary residence, the income doesn't meet the 2% rule, nor the 50% rule.
In fact, I refinanced the principal mortgage to a 20 year just to realize extra equity in the case I sell it in 3-5 years--cash flow isn't the priority for me here. If I don't sell, it'll get paid off faster. The tradeoff is ~$250 per month, which I could attribute to 'forced savings,' similar to what you're discussing with the $100 per month out of pocket.
So ask yourself, "what if?" What if that loan balance was amortized over 30 years? The difference between that 30-year PITI and your proposed PITI would be your 'forced savings.' The 50% rule would look a lot different because your loan is amortized over 20 more years.
With that said, your 'forced savings' isn't the only key factor here. That should only serve to help you frame your comfort level...if you'd rather put $100, $200, or $500 away towards this portfolio instead of an IRA or TSP, that makes sense.
However, you definitely need to do the research & make sure your numbers are solid, and that you've got cash reserves (or access to a line of credit) to tap into. Know the properties up front, major repairs/replacements that need to be made, warranties on recent repairs/replacements, expenses, leases (are they all ending within a month or two of each other--what happens if you have two or three vacancies at a time).
With seller financing, you should be able to negotiate your first year (or two) of payments based upon rental income after expenses. If the properties are solid, the seller should be willing to give this to you, knowing their rents & expenses are in line. If not, I'd be very skeptical & see what potential problems you're about to take over (deferred maintenance, lease expiration, evictions, etc.). Don't underestimate your expenses just because you're okay with putting some money out of pocket on the PITI side.
Assuming the deal works, after you've established a record of payments, you should be able to refinance at least some (if not all) of the properties to get your PITI down.
Hope this helps.
Forrest