The 50% rule includes vacancy rate; typically 5%. But ACTUAL vacancy is almost always higher than that. I use 10% in my calculations, especially if I am raising rents.
You should be able to get management done in the 4-6% range, depending on how many choices of management companies you have. (A factor in smaller towns.) For a property 30 min. from me, I'd probably want to figure on the payroll expense of having an on-site "helper" for management...eyes and ears that report to YOU, not the management company. The cost? Maybe one-half month's rent or so.
For repairs, 5% is usually my minimum budget...if you're not spending that, the building has deferred maintenance. Add another 5% for cap-ex (which, by definition, isn't an expense, so shouldn't be included in the 50%, but should be included in your cash flow analysis).
Other figures depend on your unique situation: Local property taxes (which WILL go up the year after you buy it) and insurance being prime examples. Water and sewer are significant but localized as well. Is dumpster service included in your water bill, or do you have to use a 3rd party for trash? Landscaping, janitorial and snow removal are often overlooked expenses; "because the owner does those" is often the reason the seller doesn't list them. Your expense to drive to the property once a week at $.55/mi. should be included as travel expenses (and maybe even your time). So should $500/year for tax return prep. Advertising is no longer a significant expense, since Craigs List is free, but I add a budget of 3% of the vacancy rate so that I can cover extra advertising like social media and for-rent web sites.
If you add it all up, it's really hard to be under 40% total for an older building unless it's been fully renovated and therefore has near-zero repair expenses. If it's master-metered for heat/lights/water/sewer, 50% (plus vacancy) is more realistic.
The point is to ALWAYS be conservative. Everything costs more than you think it well, and certainly more than the seller shows.
You can't "really" know what it will cost until 1.) You've seen the seller's tax returns and profit/loss statements, and 2.) You have run the property yourself for 2-3 years.
As for consulting with someone about the property, you can try a non-compete and non-disclosure agreement. But be willing to PAY for consulting.
It's great you have a lead. You are in personal contact with the seller? You've met? You've figured out why he wants to sell and what his needs are? Estimate your expenses and what you need to see for cash flow and offer accordingly. He wants $825,000?
GET ON IT. If you don't, someone else will. Don't wait to get his numbers. If his "actual" numbers vary greatly from your projections, you will have to "retrade" (reprice/re-negotiate) with the seller. But that comes later. For now, I'd offer in an LOI a couple of options: Full price with small down, lower price with more down. Closing 60 days after you receive all of the Seller's Documents. But GET IT UNDER CONTRACT this week. Don't over-analzye...move.